CursorStep
Clients are allowed to pick which step Cursor moves with.
CursorStep implementors are objects with (possibly) state
that advance cursor from one index to another, influenced
by the state mentioned and/or their own opinion on how the
motion should happen.
Instance methods
Returns the next cursor index.
prev and nxt are indices belonging to document.
prev index is the previous index (where motion started).
nxt index is the speculative, character-based next index. It might be out of document's bounds.
The latter is usually the index you'd need to change/snap,
as a CursorStep implementor. The former is given to you
so you can e.g. compute delta or determine the direction
of the movement.
The returned index must be in bounds of the document.