Configuration overlaps between Styles and other CKE5 plugins

Problem/Motivation

Follow-up of #3222797: Upgrade path from CKEditor 4's StylesCombo to CKEditor 5's Style
When using the Styles and the sourceEditing plugins there can be situation with conflicting configuration.

The main idea with CKE5 config is to not have two plugins with overlaping supported elements.

Steps to reproduce

1. Set up a CKE5 text format with a Styles config of
   

   span.test|Test
   

2. and a sourceEditing config of
   

   <span class>
   

3. An error message is shown:
   

   A style must only specify classes not supported by other plugins. The test classes on <span> are already supported by the enabled Source editing plugin.

Proposed resolution

Discuss what needs to happen in that situation.

1. Do nothing, since the user is expected to use sourceEditing to alter the span classes, the styles plugin shouldn't be able to touch those.
2. It can make sense to have the Styles plugin add classes to an element for less technical users, while still allowing the source editing plugin to allow all classes for more technical users. (Removing the validation constraints, it's working as expected in the UI). It can introduce a grey area where unexpected behaviors happens though.

   In that case the resulting Allowed HTML tags will be as followed

   <span class>
   

Remaining tasks

Agree on a solution

User interface changes

API changes

Data model changes

Release notes snippet