Add irb to Gemfile to fix warning in Ruby v4

This fixes the following warning:

    /usr/local/bundle/gems/yard-0.9.38/lib/yard/parser/ruby/legacy/irb/slex.rb:13:
    warning: irb/notifier is found in irb, which is not part of the default gems
    since Ruby 4.0.0. You can add irb to your Gemfile or gemspec to fix this
    error.

This should really be fixed by the yard gem adding a dependency on irb, but
this will do for now.

Author: floehopper

Gemfile

@@ -12,5 +12,6 @@ group :development do
     gem 'rdoc'
     gem 'redcarpet'
     gem 'yard'
+    gem 'irb'
   end
 end