[ruby/yarp] fix: %i list spanning a heredoc

The fix here is similar to what we did in a previous commit for %w, to
accept two consecutive string tokens without a separator.

https://github.com/ruby/yarp/commit/f869fbdbe5

test/yarp/fixtures/spanning_heredoc.txt

@@ -36,3 +36,9 @@ pp <<-A, %W[l\
 k
 A
 l]
+
+# ripper can't parse this successfully, though ruby runs it correctly
+pp <<-A, %i[n\
+m
+A
+n]

test/yarp/snapshots/spanning_heredoc.txt

@@ -1,6 +1,6 @@
-ProgramNode(164...709)(
+ProgramNode(164...802)(
   [],
-  StatementsNode(164...709)(
+  StatementsNode(164...802)(
     [CallNode(164...192)(
        nil,
        nil,
@@ -160,6 +160,29 @@ ProgramNode(164...709)(
        nil,
        0,
        "pp"
+     ),
+     CallNode(781...802)(
+       nil,
+       nil,
+       (781...783),
+       nil,
+       ArgumentsNode(784...802)(
+         [InterpolatedStringNode(784...788)(
+            (784...788),
+            [StringNode(796...798)(nil, (796...798), nil, "m\n")],
+            (798...800)
+          ),
+          ArrayNode(790...802)(
+            [SymbolNode(793...796)(nil, (793...796), nil, "n\\\n"),
+             SymbolNode(800...801)(nil, (800...801), nil, "n")],
+            (790...793),
+            (801...802)
+          )]
+       ),
+       nil,
+       nil,
+       0,
+       "pp"
      )]
   )
 )

yarp/yarp.c

@@ -11944,14 +11944,9 @@ parse_expression_prefix(yp_parser_t *parser, yp_binding_power_t binding_power) {
             yp_array_node_t *array = yp_array_node_create(parser, &parser->previous);
 
             while (!match_any_type_p(parser, 2, YP_TOKEN_STRING_END, YP_TOKEN_EOF)) {
-                if (yp_array_node_size(array) == 0) {
-                    accept(parser, YP_TOKEN_WORDS_SEP);
-                } else {
-                    expect(parser, YP_TOKEN_WORDS_SEP, "Expected a separator for the symbols in a `%i` list.");
-                    if (match_type_p(parser, YP_TOKEN_STRING_END)) break;
-                }
-
+                accept(parser, YP_TOKEN_WORDS_SEP);
                 if (match_type_p(parser, YP_TOKEN_STRING_END)) break;
+
                 expect(parser, YP_TOKEN_STRING_CONTENT, "Expected a symbol in a `%i` list.");
 
                 yp_token_t opening = not_provided(parser);