Fix handleEnter in nodes that don't split

ve.ce.Surface.js:
* If Enter pressed in a node that splits, work as before
* If Enter pressed in a branch slug, insert an empty paragraph
* If Enter pressed after a content offset, handle from that offset
* If Enter pressed otherwise, insert an empty paragraph

ve.ce.*Node.js:
* Rename canBeSplit to splitOnEnter
* Set splitOnEnter to true in ve.ce.ContentBranchNode (inherited by subclasses)

Bug: 59911
Change-Id: I73f85522d6fe6c22002e5f9de6c9ba6d90aef7a2
Merge "Fix handleEnter in nodes that don't split"