RichText: revise onSplit prop

[ellatrix]

We should revise the onSplit RichText prop as this is a very weird API that tries to handle too many things. If is used for both RichText splitting (on ENTER) and for paste. If it's used for paste, null may be be passed as before and after values (if pasting at the very start or end of the field), which may seem unexpected as a block implementor. Even in our own core blocks we do not always account for null being passed. Only when pasting there is a third argument with an array of blocks that may need to be inserted. Special paste handling may not always be wanted, e.g. in the list block we want pasted content to convert to list items, not cause a split and insert in between.

I think it would make more sense to separate the two, which would also make the API clearer. Still thinking on the best way to handle it, but I wanted to file this issue so it is known for API freeze.

[mcsf]

Something that this could absorb: #10326

[youknowriad]

This is marked unstable now, so I'm punting this.