Show an indicator while waiting for results [#3548673]

Problem/Motivation

For large datasets it may take 5s or more after typing until you see any visual feedback.

It would be very helpful to show a loading spinner or anything else once mentions is running.

Maybe the plugin has a built-in functionality for that or we can use callbacks to show / hide the Drupal spinner at that place or a "Loading..." entry in the list immediately and replace it with the results?
https://ckeditor.com/docs/ckeditor5/latest/features/mentions.html