Draft: Added support for custom CKEditor 5 builds:
This allows CKEditor 5 to be used with wysiwyg on Drupal 7. Both a standard build as a custom build are supported.
However, the toolbar is not (yet) configurable in the UI, but hard-coded in wysiwyg/editors/js/ckeditor5.js. Therefore, this MR is marked as draft.
See issue #3123281