build: Add irb as development dependency for Ruby 4.0

IRB was removed from Ruby 4.0's default gems and must be explicitly
declared as a dependency. This fixes the LoadError when fakefs tries
to load irb.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: highb

pathspec.gemspec

@@ -21,6 +21,7 @@ Gem::Specification.new do |s|
   s.required_ruby_version = '>= 3.2.0'
   s.add_development_dependency 'bundler', '>= 2.4.19'
   s.add_development_dependency 'fakefs', '~> 2.5'
+  s.add_development_dependency 'irb', '~> 1.14'
   s.add_development_dependency 'kramdown', '~> 2.3'
   s.add_development_dependency 'rake', '~> 13.0'
   s.add_development_dependency 'rspec', '~> 3.10'