[ruby/irb] Declare rdoc as dependency

(https://github.com/ruby/irb/pull/648)

IRB already has several features that rely on rdoc, such as:

- Autocompletion's document dialog
- Autocompletion's `PerfectMatchedProc`
- The `show_doc` command
- Easter egg

And we could use its pager more in the future too. So it makes sense to
declare rdoc as a dependency instead of relying on the one bundled with
Ruby.

https://github.com/ruby/irb/commit/4dffbb1dd3

dir.c

@@ -3266,7 +3266,7 @@ dir_open_dir(int argc, VALUE *argv)
  *   #<Encoding:UTF-8>
  *   #<Encoding:US-ASCII>
  *
- * See {String Encoding}[https://docs.ruby-lang.org/en/master/encodings_rdoc.html#label-String+Encoding].
+ * See {String Encoding}[rdoc-ref:encodings.rdoc@String+Encoding].
  *
  * Returns an enumerator if no block is given.
  */
@@ -3302,7 +3302,7 @@ dir_collect(VALUE dir)
  *   Dir.entries('/example', encoding: 'US-ASCII').first.encoding
  *   # => #<Encoding:US-ASCII>
  *
- * See {String Encoding}[https://docs.ruby-lang.org/en/master/encodings_rdoc.html#label-String+Encoding].
+ * See {String Encoding}[rdoc-ref:encodings.rdoc@String+Encoding].
  *
  * Raises an exception if the directory does not exist.
  */
@@ -3399,7 +3399,7 @@ dir_collect_children(VALUE dir)
  *   Dir.children('/example', encoding: 'US-ASCII').first.encoding
  *   # => #<Encoding:US-ASCII>
  *
- * See {String Encoding}[https://docs.ruby-lang.org/en/master/encodings_rdoc.html#label-String+Encoding].
+ * See {String Encoding}[rdoc-ref:encodings.rdoc@String+Encoding].
  *
  * Raises an exception if the directory does not exist.
  */

lib/irb/irb.gemspec

@@ -42,4 +42,5 @@ Gem::Specification.new do |spec|
   spec.required_ruby_version = Gem::Requirement.new(">= 2.7")
 
   spec.add_dependency "reline", ">= 0.3.6"
+  spec.add_dependency "rdoc", "~> 6.5"
 end