DEV: Remove dependence on dartsass-sprockets (#23665)

Discourse has a custom stylesheet pipeline which compiles things 'just in time'. The only place we were still running sass files through sprockets was for the `/tests` route in development mode. This use can be removed by compiling the relevant stylesheets through ember-cli instead (which we were already doing for testem runs)

This work was prompted by the incompatibility of dartsass-sprockets with the latest sass-embedded release (https://github.com/tablecheck/dartsass-sprockets/issues/13)

config/application.rb

@@ -181,6 +181,15 @@ module Discourse
                                  charset: :unicode
     Sprockets.register_postprocessor "application/javascript", DiscourseJsProcessor
 
+    class SprocketsSassUnsupported
+      def self.call(*args)
+        raise "Discourse does not support compiling scss/sass files via Sprockets"
+      end
+    end
+
+    Sprockets.register_engine(".sass", SprocketsSassUnsupported, silence_deprecation: true)
+    Sprockets.register_engine(".scss", SprocketsSassUnsupported, silence_deprecation: true)
+
     Discourse::Application.initializer :prepend_ember_assets do |app|
       # Needs to be in its own initializer so it runs after the append_assets_path initializer defined by Sprockets
       app