Add new variable 'forward-list-function' for 'treesit-forward-list'

Add new variable 'forward-list-function' for 'treesit-forward-list'

* lisp/emacs-lisp/lisp.el (forward-list-default-function): New function.
(forward-list-function): New variable (bug#73404).
(forward-list): Move meat to 'forward-list-default-function',
and call 'forward-list-function' when non-nil.

* lisp/treesit.el (treesit-forward-list): Rewrite to not rely on
'treesit-forward-sexp'.
(treesit-major-mode-setup): Set 'forward-list-function' to
'treesit-forward-list'.

(cherry picked from commit 3db984c72b8609c84f06a0fd62e59b4823ea0876)