New variable 'comment-setup-function' for multi-language modes.

New variable 'comment-setup-function' for multi-language modes.

* lisp/newcomment.el (comment-setup-function): New variable.
(comment-normalize-vars): Call non-nil 'comment-setup-function'.

* lisp/textmodes/mhtml-ts-mode.el
(mhtml-ts-mode--comment-current-lang): New internal variable.
(mhtml-ts-mode--comment-setup): New function.
(mhtml-ts-mode): Set 'comment-setup-function' to
'mhtml-ts-mode--comment-setup' instead of using
'c-ts-common-comment-setup' only for JavaScript.

https://lists.gnu.org/archive/html/emacs-devel/2025-05/msg00025.html
(cherry picked from commit 61cb73a2dbe756b060d4256c3b7ebe09f71ed2b7)