fix: advance cursor during cmd-transpose

[joelim-work]

The cmd-transpose command should advance the cursor after swapping characters, similar to how GNU Readline works:

transpose-chars (C-t)
Drag the character before point forward over the character
at point, moving point forward as well. If point is at the
end of the line, then this transposes the two characters
before point. Negative arguments have no effect.

The implementation has also been changed to handle entire grapheme clusters as well.