[UX][A11y][D10] CKEditor toolbar configuration: allow adding/(re)moving buttons via keyboard

I was just comparing the CKEditor configuration form in Drupal, as part of assisting @quicksketch with various issues with the work being done over in https://github.com/quicksketch/ckeditor5/issues to move #4122 forward. I noticed that the help text in the respective form in Drupal has the following help text:

Move a button into the Active toolbar to enable it, or into the list of Available buttons to disable it. Buttons may be moved with the mouse or keyboard arrow keys.

I have tested this, and it works quite nicely/intuitively! Once you have tabbed your way to the first toolbar button in the list of available ones, you can: - press the key down button on the keyboard to move the currently selected button from the available to the active ones (it is moved to the end of the other buttons in the active toolbar) - press the left/right keys to reorder the button - press tab or shift+tab to select the next/previous button to the currently selected button - press the up key to move the selected button back to the disabled/available list

Here's a screencast of that in action:

The current plan is to add CKEditor 5 as an additional core module and have that be the default for new sites, while keeping v4 for existing sites and allowing them to move to v5 when ready. While we are supporting both CKEditor 4 and 5 (and I appreciate the fact that v4 will become EoL by the end of the year), we should aim to add keyboard navigation to both versions.