CKEditor 5 compatibility

Unfortunately CKEditor strips out empty inline tags, even in source mode. See the following:

• 🐛 [upstream] [GHS] CKEditor 5 does not retain custom HTML tags that are not defined by CKEditor 5 plugins whenever /.*/ is not allowed (e.g. when filter_html is enabled) Postponed
• 🐛 [upstream] [GHS] CKEditor 5 removes empty inline elements Fixed
• https://github.com/ckeditor/ckeditor5/issues/9888

It's possible that we could provide a CKeditor plugin to resolve this, and/or investigate further how the <drupal-media> tags are retained.