List v2: splitting/merging

[ellatrix]

It should be possible to split list items (use RichText's onSplit).
When splitting an empty list item, the list as a whole should split.

It should also be possible to merge list items (use RichText's onMerge and and merge in the block type).
It should be possible to merge a paragraph into a list, but this could be done separately as it will require #39520.

[ellatrix]

This is done. Created #39877 to refine.

[ntsekouras]

This still needs to handle merge on blocks with innerBlocks that involve RichText API. It's not working as expected right now for example if you press Enter on a list item with innerBlocks.

[ellatrix]

It seems to work? Let's create new issues for polish it.

[ellatrix]

Opened #42499.

[htmgarcia]

This still needs to handle merge on blocks with innerBlocks that involve RichText API. It's not working as expected right now for example if you press Enter on a list item with innerBlocks.

@ntsekouras I'm curious if there are any docs explaining how to add this behavior to innerBlocks: insert a new block when pressing enter