Draft: Plain / Rich text editor full screen enhancements

What does this MR do and why?

Turns the existing "zen-mode" in the plain text editor into a "full-screen mode" for both the plain and rich text editors. This pulls in the entire editors, including the toolbars.

This currently works with the original pre-panel architecture. However the new panel architecture introduces enough changes so that a different approach will be needed.

References

Related to #343605

Screenshots or screen recordings

Working draft

How to set up and validate locally

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