Support ckeditor_stylesheets setting in theme info, allow CSS to work for both CKEditor 4 and 5

Problem/Motivation

The ckeditor_stylesheets property in a theme's info.yml makes it possible to CKEditor contents with CSS from the front-end theme. More info on ckeditor_stylesheets use here.

This also needs to be supported in the CKEditor 5 module, and in a way that applies the styles from ckeditor_stylesheets only to CKEditor, not the full page.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes