API addition: \Drupal\editor\Plugin\EditorPluginInterface::getDefaultSettings() should accept old Editor + Format to generate smart defaults

Problem/Motivation

By working on #3216015: Generate CKEditor 5 configuration based on pre-existing text format configuration for CKEditor 5, we noticed a lot of hoops to jump through.

When switching from CKEditor 4 to CKEditor 5, or just when enabling CKEditor 5, we should give the user (the site builder) a sensible result when they see CKEditor 5:

1. When switching from CKEditor 4 to CKEditor 5, they should have an identical toolbar layout if at all possible
2. When enabling CKEditor 5 for a text format that did not yet have a text editor configured, it should not just enable a CKEditor 5 toolbar configuration of Heading Bold Italic — it should automatically place toolbar items that are relevant for the allowed HTML tags!

But the current \Drupal\editor\Plugin\EditorPluginInterface::getDefaultSettings() API makes that impossible, because it is not given any context. It is currently a poorly designed API (I can say those words because I created it ~7 years ago 🙈) because it could currently have been static.

Steps to reproduce

See above.

Proposed resolution

Take advantage of the fact that \Drupal\editor\Plugin\EditorPluginInterface::getDefaultSettings() is not static right now, and add optional parameters:

\Drupal\editor\Plugin\EditorPluginInterface::getDefaultSettings(FilterFormatInterface $text_format, EditorInterface $old_text_editor)

That would allow it to not return a hardcoded default, but a contextually relevant default.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet