Adapt recent css-ts-mode changes to mhtml-ts-mode

Adapt recent css-ts-mode changes to mhtml-ts-mode

* lisp/textmodes/css-mode.el (css--treesit-thing-settings):
(css-ts-mode--outline-predicate): New variables.
(css-ts-mode): Set treesit-outline-predicate.

* lisp/textmodes/mhtml-ts-mode.el (mhtml-ts-mode--treesit-thing-settings):
Use css--treesit-thing-settings and css--treesit-defun-type-regexp.
(mhtml-ts-mode): Use 'css-ts-mode--outline-predicate' for
'treesit-aggregated-outline-predicate'.

(cherry picked from commit 888a2b5394f4574edd5fe6e76bef91aab8111ebd)