DEV: Include lib/stylesheet mtime in stylesheet cache (#31063)

Followup 23edfe7cc2

When working on changes to any of the lib/stylesheet/*.rb
files, the color_definitions.scss and other stylesheet
caches can be annoyingly persistent.

This commit adds all of these lib files to the `max_file_mtime`
part of the `fs_asset_cachebuster` cache key, so if any of
them are changed then the cache will break, making development
a lot easier.

lib/stylesheet/manager.rb

@@ -172,6 +172,7 @@ class Stylesheet::Manager
     globs = [
       "#{Rails.root}/app/assets/stylesheets/**/*.*css",
       "#{Rails.root}/app/assets/images/**/*.*",
+      "#{Rails.root}/lib/stylesheet/*.rb",
     ]
 
     Discourse.plugins.each do |plugin|