New thing-at-point target 'string' used in context-menu-region

New thing-at-point target 'string' used in context-menu-region

* lisp/mouse.el (context-menu-region): Use separate "List" and "String".

* lisp/thingatpt.el (string): New target 'string'.
(thing-at-point-bounds-of-string-at-point): New function.
(thing-at-point-bounds-of-list-at-point): Revert previous commit
by removing optional args 'escape-strings' and 'no-syntax-crossing'.
(list-or-string): Remove recently added target 'list-or-string'.
(thing-at-point-bounds-of-list-or-string-at-point): Remove function.
https://lists.gnu.org/archive/html/emacs-devel/2021-09/msg01737.html