RichText: stabilize onSplit

[ellatrix]

Description

Stablizes the RichText onSplit prop.

• Fixes RichText: revise onSplit prop #10761, see the issue for more information.
• Fixes Using Enter to split a paragraph block requires two Ctrl-Z's to undo #8882: Using Enter to split a paragraph block requires two Ctrl-Z's to undo.
• Fixes Pressing enter at the start of a heading block should push it down and create a paragraph above. #10326. Pressing enter at the start of a heading block should push it down and create a paragraph above.
• Fixes Clicking "Enter" at the beginning of text bumps it down, but loses the block styling #13740. Keep paragraph styling on split at the start.
• Fixes pasting a list inside a list.
• It also fixes an issue with lists instances where pasted content would split the block. Since these two are now separate handlers, pasted content can now appear as list items inside lists.

Additionally it fixes issues where core blocks (heading and list) do not take into account any null values passed to unstableOnSplit.

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.

[ellatrix]

@youknowriad I can't reproduce that issue. What browser are you using?

[youknowriad]

@ellatrix I'm using Firefox

[ellatrix]

@youknowriad It looks like it's a visual glitch that happens in master as well.

[youknowriad]

@ellatrix I confirm, let's fix this bug separately.

[ellatrix]

@youknowriad Addressed your feedback. Additionally, #14846 seems to fix the issue you're having in Firefox.

[youknowriad]

Great work

[ellatrix]

Thanks @youknowriad!