DEV: Improve strategy for identifying ember-cli JS chunks (#23336)

Our Ember build compiles assets into multiple chunks. In the past, we used the output from `ember-auto-import-chunks-json-generator` to give Rails a map of those chunks. However, that addon is specific to ember-auto-import, and is not compatible with Embroider.

Instead, we can switch to parsing the html files which are output by ember-cli. These are guaranteed to have the correct JS files in the correct place. A `<discourse-chunked-script>` will allow us to easily identify which chunks belong to which entrypoint.

In future, as we update more entrypoints to be compiled by Embroider/Webpack, we can easily introduce new wrappers.

app/helpers/application_helper.rb

@@ -133,10 +133,12 @@ module ApplicationHelper
   end
 
   def preload_script(script)
-    scripts = [script]
+    scripts = []
 
     if chunks = EmberCli.script_chunks[script]
       scripts.push(*chunks)
+    else
+      scripts.push(script)
     end
 
     scripts