Add mhtml-ts-mode.

Add mhtml-ts-mode.

New major-mode alternative to mhtml-mode, based on treesitter, for
editing files containing html, javascript and css.

* etc/NEWS: Mention the new mode and new functions.
* lisp/textmodes/mhtml-ts-mode.el: New file.
* lisp/progmodes/js.el
(js--treesit-thing-settings): New variable.
(js--treesit-font-lock-feature-list); New variable.
(js--treesit-simple-imenu-settings): New variable.
(js--treesit-defun-type-regexp): New variable.
(js--treesit-jsdoc-comment-regexp): New variable.
(js-ts-mode): Use of new variables instead of direct assignment of
values.
* lisp/textmodes/css-mode.el
(css-mode--menu): New variable.
(css-mode-map): Use new variable.
(css--treesit-font-lock-feature-list): New variable.
(css--treesit-simple-imenu-settings): New variable.
(css--treesit-defun-type-regexp): New variable.
(cs-ts-mode): Use of new variables instead of direct assignment of
values.
* lisp/textmodes/html-ts-mode.el
(html-ts-mode--treesit-things-settings): New variable.
(html-ts-mode--treesit-font-lock-feature-list): New variable.
(html-ts-mode--treesit-simple-imenu-settings): New variable.
(html-ts-mode--treesit-defun-type-regexp): New variable.
(html-ts-mode): Use of new variables instead of direct assignment of
values.
* lisp/treesit.el
(treesit-merge-font-lock-feature-list): New fuction.
(treesit-replace-font-lock-feature-settings): New fuction.
(treesit-modify-indent-rules): New function.

(cherry picked from commit 05a96fd39809f11a3820e2164b23ebf9df192b13)