Allow <details> element to be placed without <div>

The patch seems to change the indentation, which makes it difficult to see what code changes are actually being proposed. Can we get an updated patch that doesn't change the indentation? Also, it would be useful to know if the same changes are needed for the CKEditor5 version of the plugin introduced in the 2.1.x branch.

Patch for CKEditor 5 (only) support

I've tested 3244311-6.patch, running into a few issues.

The order of the elements changes on load of the editor.
To reproduce:

1. Place a details element in the editor
2. Add multiple elements (<p>) to it
3. Change to 'source editing'
4. Add some characters
5. Change back to normal editing
6. See the order of your items has changed.

Video demonstrating the issue → (also attached)

Some elements get stripped from the content of the details, such as heading elements.
To reproduce:

1. Place a details element in the editor
2. Add a heading in the content of the editor
3. Change to 'source editing'
4. Add some characters
5. Change back to normal editing
6. See the heading has been converted to a <p>

Video demonstrating the issue → (also attached)

Please note that this not only happens when switching from source editing to regular editing, also when re-opening the edit page after saving. Without switching in between the values in the database are 'correct' but get swapped when opening the editor again. Headings are still allowed, just disappear when opening the editor again.