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

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

* lisp/emacs-lisp/lisp.el (down-list-default-function): New function.
(down-list-function): New variable (bug#73404).
(down-list): Move meat to 'down-list-default-function',
and call 'down-list-function' when non-nil.  Don't raise an error
in strings or comments when 'down-list-function' is non-nil.

* lisp/treesit.el (treesit--scan-error): New internal function.
(treesit-forward-sexp, treesit-forward-list): Use 'treesit--scan-error'.
(treesit-down-list): New function.
(treesit-major-mode-setup): Set 'down-list-function' to
'treesit-down-list'.

(cherry picked from commit 3c50edb2b500c6ac18696e99c3f8df597dea54d4)