(smerge-refine-shadow-cursor): New variable and face (bug#78806)

(smerge-refine-shadow-cursor): New variable and face (bug#78806)

* lisp/vc/smerge-mode.el (smerge-refine-shadow-cursor): New variable
and face.
(smerge-refine-regions): Add 'cursor-sensor-functions' property
to the covering overlays.
(smerge--refine-at-right-margin-p, smerge--refine-shadow-cursor):
New functions.
(smerge--refine-other-pos): New function, extracted from
smerge-refine-exchange-point'.
(smerge-refine-exchange-point): Use it.
(smerge--refine-highlight-change): Add thin
highlighted space for insertion/deletion positions.

* lisp/emacs-lisp/cursor-sensor.el (cursor-sensor--detect):
Run functions for 'moved' events.  Demote errors.
(cursor-sensor-mode): Adjust docstring accordingly.

* doc/lispref/text.texi (Special Properties) <cursor-sensor-functions>:
Mention the new 'moved' direction.

(cherry picked from commit 08fba517f66794f817c3e6101a80ce3df1798581)