RichText: set correct selection on split

[ellatrix]

Description

Still a work in progress.

This PR makes it so that RichText sets the correct position selection when a block is split by RichText so the selection is directly set in the store instead of the block trying to query the first tabbable element inside the block (which may not be the element that has the split text). The technique is similar to the MERGE_BLOCKS effect.

I'll see if I can fix the selection on paste as well.

How has this been tested?

Screenshots

Types of changes

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR. .