Add debug gem

This is included in new rails 7+ apps, so let's include it too.

This also means it's available on ruby 3.0, and is automatically
updated on newer versions of ruby too.

Gemfile

@@ -180,4 +180,7 @@ end
 
 group :development, :test do
   gem "annotate"
+
+  # See https://guides.rubyonrails.org/debugging_rails_applications.html#debugging-with-the-debug-gem
+  gem "debug", :require => "debug/prelude"
 end