DEV: Make problem check registration more explicit (#26413)

Previously the problem check registry simply looked at the subclasses of ProblemCheck. This was causing some confusion in environments where eager loading is not enabled, as the registry would appear empty as a result of the classes never being referenced (and thus never loaded.)

This PR changes the approach to a more explicit one. I followed other implementations (bookmarkable and hashtag autocomplete.) As a bonus, this now has a neat plugin entry point as well.

lib/plugin/instance.rb

@@ -317,6 +317,10 @@ class Plugin::Instance
     Site.preloaded_category_custom_fields << field
   end
 
+  def register_problem_check(klass)
+    DiscoursePluginRegistry.register_problem_check(klass, self)
+  end
+
   def custom_avatar_column(column)
     reloadable_patch do |plugin|
       UserLookup.lookup_columns << column