TinyMCE: Clean TinyMCE internal DOM state on blur

[aduth]

Fixes #8877

In Progress: Still needs unit tests, documentation, and evaluation of whether this is the best approach. Other options include calling setContent from RichText to just replace the content, though this may be more intensive since it requires TinyMCE to re-parse. There may be room for improvement to refactor shared behaviors between "dom" and "content" formats in formats.js. Also not sure we want to expose this as a proper format for RichText.

This pull request seeks to resolve an issue where lingering TinyMCE DOM state when a RichText field is blurred can occasionally cause forced re-focuses.

Testing instructions:

Repeat steps to reproduce from #8877, verifying that caret placement occurs as expected.

Ensure end-to-end tests pass:

npm run test-e2e

[aduth]

Related: Implementing some of the ideas with on-demand TinyMCE in #8822 / #341 may require the same sort of "clean-up" to occur when leaving a RichText field.

[ellatrix]

Closing this PR as #8877 is fixed.