Fix YARD compatibility with Ruby 4.0 by adding irb dependency

Author: rubiii

Gemfile

@@ -9,6 +9,9 @@ gem 'syslog' if RUBY_VERSION >= '3.4'
 # benchmark was removed from default gems in Ruby 4.0 (silence warning on 3.4+)
 gem 'benchmark' if RUBY_VERSION >= '3.4'
 
+# irb was removed from default gems in Ruby 4.0 (required by yard's legacy parser)
+gem 'irb' if RUBY_VERSION >= '4.0'
+
 group :development do
   gem 'benchmark-ips', '~> 2.14', require: false
   gem 'equivalent-xml', '~> 0.6'