Register CKEditor 5 plugins via YAML definitions instead of class annotations

Rebased this after adding the image plugins in #3194111: Support inline image upload
Moved some of the code for getDefinitions out of the Editor plugin so that a plugin class can override how those definitions are returned. ImageUpload is split out into its own plugin now and just provides info for that feature.
The toolbar is working, text only still but toolbar items can be selected and sorted here.

Updated the test module to use the yml plugin and the toolbar settings.

Discussed with Peter and decided to keep annotations, and just allow YAML in addition. This brings it inline with other core plugin systems

This is a big refactor to add yml as an optional way of defining the ckeditor5 config each Drupal plugin provides.
This should greatly simplify adding a contrib module, since it now needs to just declare the js classes that are needed for the functionality, as well as the available toolbar items and any additional config necessary.

A php class can still be written for a plugin that depends on editor settings, so it may conditionally add additional values or properties that are dynamically set by Drupal.

This MR also adds the beginnings of the toolbar config, currently just a text field but it allows a user to select which toolbar buttons should be shown and in what order.