Add CKEditor 5 module to Drupal core

The easy questions to answer: Why? Why switch from CKEditor 4? It’s approaching its end-of-life in 2023. Why CKEditor 5 and not something else? Successful past collaboration, healthy project, clearly significantly better than its predecessor, and also: time constraint. See the core policy issue where this was decided. Why now? The CKEditor 5 module needs to be stable in the last Drupal 9 minor if we want to be able to deprecate CKEditor 4 (ckeditor.module) before Drupal 10, so we can remove CKEditor 4 in Drupal 10.