[ruby/prism] Fix token incompatibility for `Prism::Translation::Parser::Lexer`

This PR fixes token incompatibility for `Prism::Translation::Parser::Lexer` when using backquoted heredoc indetiner:

```ruby
<<-`  FOO`
a
b
     FOO
```

## Parser gem (Expected)

Returns `tXSTRING_BEG` as the first token:

```console
$ bundle exec ruby -Ilib -rparser/ruby33 -ve \
'buf = Parser::Source::Buffer.new("example.rb"); buf.source = File.read("example.rb"); p Parser::Ruby33.new.tokenize(buf)'
ruby 3.3.0 (2023-12-25 revision https://github.com/ruby/prism/commit/5124f9ac75) [x86_64-darwin22]
[s(:xstr,
  s(:str, "a\n"),
  s(:str, "b\n")), [], [[:tXSTRING_BEG, ["<<`", #<Parser::Source::Range example.rb 0...10>]],
[:tSTRING_CONTENT, ["a\n", #<Parser::Source::Range example.rb 11...13>]],
[:tSTRING_CONTENT, ["b\n", #<Parser::Source::Range example.rb 13...15>]],
[:tSTRING_END, ["  FOO", #<Parser::Source::Range example.rb 15...23>]], [:tNL, [nil, #<Parser::Source::Range example.rb 10...11>]]]]
```

## `Prism::Translation::Parser` (Actual)

Previously, the tokens returned by the Parser gem were different. The escaped backslash does not match in the `tSTRING_BEG` token and
value of `tSTRING_END` token.

```console
$ bundle exec ruby -Ilib -rprism -rprism/translation/parser33 -ve \
'buf = Parser::Source::Buffer.new("example.rb"); buf.source = File.read("example.rb"); p Prism::Translation::Parser33.new.tokenize(buf)'
ruby 3.3.0 (2023-12-25 revision https://github.com/ruby/prism/commit/5124f9ac75) [x86_64-darwin22]
[s(:xstr,
  s(:str, "a\n"),
  s(:str, "b\n")), [], [[:tSTRING_BEG, ["<<\"", #<Parser::Source::Range example.rb 0...10>]],
[:tSTRING_CONTENT, ["a\n", #<Parser::Source::Range example.rb 11...13>]],
[:tSTRING_CONTENT, ["b\n", #<Parser::Source::Range example.rb 13...15>]],
[:tSTRING_END, ["`  FOO`", #<Parser::Source::Range example.rb 15...23>]], [:tNL, [nil, #<Parser::Source::Range example.rb 10...11>]]]]
```

After this correction, the AST and tokens returned by the Parser gem are the same:

```console
$ bunlde exec ruby -Ilib -rprism -rprism/translation/parser33 -ve \
'buf = Parser::Source::Buffer.new("example.rb"); buf.source = File.read("example.rb"); p Prism::Translation::Parser33.new.tokenize(buf)'
ruby 3.3.0 (2023-12-25 revision https://github.com/ruby/prism/commit/5124f9ac75) [x86_64-darwin22]
[s(:xstr,
  s(:str, "a\n"),
  s(:str, "b\n")), [], [[:tXSTRING_BEG, ["<<`", #<Parser::Source::Range example.rb 0...10>]],
[:tSTRING_CONTENT, ["a\n", #<Parser::Source::Range example.rb 11...13>]],
[:tSTRING_CONTENT, ["b\n", #<Parser::Source::Range example.rb 13...15>]],
[:tSTRING_END, ["  FOO", #<Parser::Source::Range example.rb 15...23>]], [:tNL, [nil, #<Parser::Source::Range example.rb 10...11>]]]]
```

https://github.com/ruby/prism/commit/308f8d85a1

lib/prism/translation/parser/lexer.rb

@@ -278,7 +278,7 @@ module Prism
               value = nil
             when :tSTRING_BEG
               if token.type == :HEREDOC_START
-                heredoc_identifier_stack.push(value.match(/<<[-~]?["']?(?<heredoc_identifier>.*?)["']?\z/)[:heredoc_identifier])
+                heredoc_identifier_stack.push(value.match(/<<[-~]?["'`]?(?<heredoc_identifier>.*?)["'`]?\z/)[:heredoc_identifier])
               end
               if ["\"", "'"].include?(value) && (next_token = lexed[index][0]) && next_token.type == :STRING_END
                 next_location = token.location.join(next_token.location)
@@ -294,7 +294,12 @@ module Prism
                 index += 2
               elsif value.start_with?("<<")
                 quote = value[2] == "-" || value[2] == "~" ? value[3] : value[2]
-                value = "<<#{quote == "'" || quote == "\"" ? quote : "\""}"
+                if quote == "`"
+                  type = :tXSTRING_BEG
+                  value = "<<`"
+                else
+                  value = "<<#{quote == "'" || quote == "\"" ? quote : "\""}"
+                end
               end
             when :tSTRING_CONTENT
               unless (lines = token.value.lines).one?

test/prism/fixtures/heredocs_leading_whitespace.txt

@@ -8,6 +8,11 @@ a
 b
      FOO
 
+<<-`  FOO`
+a
+b
+     FOO
+
 <<-'  FOO'
 a
 b

test/prism/snapshots/heredocs_leading_whitespace.txt

@@ -1,8 +1,8 @@
-@ ProgramNode (location: (1,0)-(21,10))
+@ ProgramNode (location: (1,0)-(26,10))
 ├── locals: []
 └── statements:
-    @ StatementsNode (location: (1,0)-(21,10))
+    @ StatementsNode (location: (1,0)-(26,10))
-    └── body: (length: 5)
+    └── body: (length: 6)
         ├── @ StringNode (location: (1,0)-(1,10))
         │   ├── flags: ∅
         │   ├── opening_loc: (1,0)-(1,10) = "<<-'  FOO'"
@@ -15,41 +15,47 @@
         │   ├── content_loc: (7,0)-(9,0) = "a\nb\n"
         │   ├── closing_loc: (9,0)-(10,0) = "     FOO\n"
         │   └── unescaped: "a\nb\n"
-        ├── @ StringNode (location: (11,0)-(11,10))
+        ├── @ XStringNode (location: (11,0)-(11,10))
         │   ├── flags: ∅
-        │   ├── opening_loc: (11,0)-(11,10) = "<<-'  FOO'"
+        │   ├── opening_loc: (11,0)-(11,10) = "<<-`  FOO`"
         │   ├── content_loc: (12,0)-(14,0) = "a\nb\n"
-        │   ├── closing_loc: (14,0)-(15,0) = "  FOO\n"
+        │   ├── closing_loc: (14,0)-(15,0) = "     FOO\n"
         │   └── unescaped: "a\nb\n"
-        ├── @ InterpolatedStringNode (location: (16,0)-(16,10))
+        ├── @ StringNode (location: (16,0)-(16,10))
-        │   ├── opening_loc: (16,0)-(16,10) = "<<~'  FOO'"
+        │   ├── flags: ∅
+        │   ├── opening_loc: (16,0)-(16,10) = "<<-'  FOO'"
+        │   ├── content_loc: (17,0)-(19,0) = "a\nb\n"
+        │   ├── closing_loc: (19,0)-(20,0) = "  FOO\n"
+        │   └── unescaped: "a\nb\n"
+        ├── @ InterpolatedStringNode (location: (21,0)-(21,10))
+        │   ├── opening_loc: (21,0)-(21,10) = "<<~'  FOO'"
         │   ├── parts: (length: 2)
-        │   │   ├── @ StringNode (location: (17,0)-(18,0))
+        │   │   ├── @ StringNode (location: (22,0)-(23,0))
         │   │   │   ├── flags: ∅
         │   │   │   ├── opening_loc: ∅
-        │   │   │   ├── content_loc: (17,0)-(18,0) = "a\n"
+        │   │   │   ├── content_loc: (22,0)-(23,0) = "a\n"
         │   │   │   ├── closing_loc: ∅
         │   │   │   └── unescaped: "a\n"
-        │   │   └── @ StringNode (location: (18,0)-(19,0))
+        │   │   └── @ StringNode (location: (23,0)-(24,0))
         │   │       ├── flags: ∅
         │   │       ├── opening_loc: ∅
-        │   │       ├── content_loc: (18,0)-(19,0) = "b\n"
+        │   │       ├── content_loc: (23,0)-(24,0) = "b\n"
         │   │       ├── closing_loc: ∅
         │   │       └── unescaped: "b\n"
-        │   └── closing_loc: (19,0)-(20,0) = "     FOO\n"
+        │   └── closing_loc: (24,0)-(25,0) = "     FOO\n"
-        └── @ InterpolatedStringNode (location: (21,0)-(21,10))
+        └── @ InterpolatedStringNode (location: (26,0)-(26,10))
-            ├── opening_loc: (21,0)-(21,10) = "<<~'  FOO'"
+            ├── opening_loc: (26,0)-(26,10) = "<<~'  FOO'"
             ├── parts: (length: 2)
-            │   ├── @ StringNode (location: (22,0)-(23,0))
+            │   ├── @ StringNode (location: (27,0)-(28,0))
             │   │   ├── flags: ∅
             │   │   ├── opening_loc: ∅
-            │   │   ├── content_loc: (22,0)-(23,0) = "a\n"
+            │   │   ├── content_loc: (27,0)-(28,0) = "a\n"
             │   │   ├── closing_loc: ∅
             │   │   └── unescaped: "a\n"
-            │   └── @ StringNode (location: (23,0)-(24,0))
+            │   └── @ StringNode (location: (28,0)-(29,0))
             │       ├── flags: ∅
             │       ├── opening_loc: ∅
-            │       ├── content_loc: (23,0)-(24,0) = "b\n"
+            │       ├── content_loc: (28,0)-(29,0) = "b\n"
             │       ├── closing_loc: ∅
             │       └── unescaped: "b\n"
-            └── closing_loc: (24,0)-(25,0) = "  FOO\n"
+            └── closing_loc: (29,0)-(30,0) = "  FOO\n"