This adds ckeditor5 as a usable Wysiwig in CiviCRM. Ckeditor5 is being actively developed while CkEditor4 is in LTS stage so it makes more sense to invest time in ckeditor5 than ckeditor4. However both have gaps. Ckeditor5 is better than ckeditor4 when it comes to inserting formatted text from word / google (note that you no longer see the paste from Word button as you just paste).
Also note ckeditor5 always uses an expandable window so the field will be small if it has no data.
This implementation gets away from the kcfinder file manager which was last updated in 2014. KcFinder was brittle around paths - especially for sites with non-standard layouts like symlinks or Drupal8.
Currently the packaged version comes with 2 options - embedded images is useful for sites operating behind a firewall as it embeds any uploaded images into the html
(as a base64 blob), meaning that images embedded in emails will not be inaccessible to the recipients. For example a user who is sending an email from CiviCRM from behind a firewall who wants to add a signature cannot use the traditional ckeditor4 image insert because the image will be located behind the firewall and inaccessible to the reader. By embedding it it will be available.
Note - there are discrepancies between email clients and some might suppress or mis-display embedded images, as some might suppress linked ones. I also hope to support elfinder
integration as an alternative inn future
The second option uses ElFinder is also an option for using uploaded files. I've labeled it experimental because it has some gaps. Intriguingly ElFinder also supports google drive, dropbox or Onedrive as backend file systems. This is not currently enabled and requires some credentials etc.