racc has been extracted to a gem since Ruby 3.3

see: https://bugs.ruby-lang.org/issues/19702

This fixes CI failures on Ruby 3.3 with the following error:
cannot load such file -- racc/parser (LoadError)

e.g.
https://github.com/ruby-i18n/i18n/actions/runs/8179356458/job/22365256416

Author: amatsuda

Gemfile

@@ -7,3 +7,4 @@ gem 'test_declarative', '0.0.6'
 gem 'rake', '~> 13'
 gem 'minitest', '~> 5.14'
 gem 'json'
+gem 'racc'