[1.1.0] Add Ckeditor5 support

Problem/Motivation

The ckeditor_stylesheets property in a theme's info.yml makes it possible to CKEditor 4 contents with CSS from the front-end theme.

ui_suite_dsfr is currently using this key:

ckeditor_stylesheets:
  - dist/dsfr.min.css

To make it works with CKEditor 5, there are 2 changes:

• the key is now ckeditor5-stylesheets
• those styles need to be written differently since CKEditor 5 is not loaded in an iframe, with a .ck-content selector prefix

We keep both ckeditor 4 and ckeditor 5 while Drupal 9 is till supported (until

Remaining tasks

• Decide how much of this CSS should be avaiable for CKEditor 5. The full dsfr.min.css seems overkill.
• Find a way of automatically prefixing all those selectors.