Move functionality into ckeditor5_plugin_pack

Problem/Motivation

See https://www.drupal.org/project/ckeditor5_plugin_pack/issues/3488849 ✨ Add Drupal insert button (teaser break) Active

Maybe we could then also get rid of the fakeobjects dependency?

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes