An implementation of an undo/redo history for ProseMirror. This
history is _selective_, meaning it does not just roll back to a
previous state but can undo some changes while keeping other, later
changes intact. (This is necessary for collaborative editing, and
comes up in other situations as well.)

@history

@undo

@redo

@undoNoScroll

@redoNoScroll

@undoDepth

@redoDepth

@closeHistory
