Rich Text: Remove createUndoLevel for split, merge

[aduth]

#4956 (comment)

This pull request seeks to optimize RichText by removing the creation of undo levels at split and merge steps. These have the adverse effect of causing the field's onChange callback to be called needlessly.

These changes introduce new, very basic end-to-end tests to verify against regression in undo behaviors on splitting.

Testing instructions:

Verify there are no regressions in splitting or merging blocks (e.g. paragraph), and that undo-ing still has the desired effect.

Ensure end-to-end tests pass:

npm run test-e2e

[jorgefilipecosta]

This is testing well for me on the split case. On the merge case, I think we still need a correction.
I did the following:
Created a paragraph with some content.
Pressed enter many times to create many empty paragraphs.
I deleted some paragraphs by pressing backspace.
I verified we need to press undo multiple times to undo each block deletion.

This behavior is not a regression It is the same way in master.

[aduth]

@jorgefilipecosta Given that the behavior is the same in master, cool if we file an issue and tackle separately, merging this one as-is?

[jorgefilipecosta]

I did not notice any regression and the changes look good 👍

[ellatrix]

Looks good to me too.