Ruby 3.5 moves readline as bundled gem

This commit addresses the following error:
```ruby
% ruby -v
ruby 3.5.0dev (2025-03-11T00:00:47Z master 47c3ae6962) +PRISM [arm64-darwin24]
~/src/github.com/rails/thor % vi GE
% bundle exec rspec ./spec/line_editor_spec.rb

An error occurred while loading ./spec/line_editor_spec.rb. - Did you mean?
                    rspec ./spec/line_editor/basic_spec.rb

Failure/Error: require "readline"

LoadError:
  cannot load such file -- readline
No examples found.

Finished in 0.00002 seconds (files took 0.18631 seconds to load)
0 examples, 0 failures, 1 error occurred outside of examples

Coverage report generated for RSpec to /Users/yahonda/src/github.com/rails/thor/coverage.
Line Coverage: 41.55% (920 / 2214)
[Coveralls] Outside the CI environment, not sending data.
Stopped processing SimpleCov as a previous error not related to SimpleCov has been detected
```

Refer to https://bugs.ruby-lang.org/issues/20309

Author: yahonda

Gemfile

@@ -16,6 +16,7 @@ group :test do
   gem "simplecov", ">= 0.13"
   gem "webmock", ">= 3.14"
   gem "rdoc"
+  gem "readline"
 end
 
 gemspec