Declare rdoc as dependency

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.

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