New variable 'treesit-aggregated-outline-predicate' (bug#76398)

New variable 'treesit-aggregated-outline-predicate' (bug#76398)

* doc/lispref/modes.texi (Outline Minor Mode):
Add 'treesit-aggregated-outline-predicate'.

* lisp/treesit.el (treesit-aggregated-outline-predicate):
New buffer-local variable.
(treesit-outline--at-point):
Use 'treesit-aggregated-outline-predicate'.
(treesit-closest-parser-boundary): New function.
(treesit-outline-search): Use 'treesit-aggregated-outline-predicate'
and 'treesit-closest-parser-boundary'.
(treesit-outline-level): Use 'treesit-aggregated-outline-predicate'.
(treesit-major-mode-setup): Add 'treesit-aggregated-outline-predicate'.

* lisp/textmodes/html-ts-mode.el (html-ts-mode--outline-predicate):
Improve.

* lisp/textmodes/mhtml-ts-mode.el (mhtml-ts-mode):
Set 'treesit-aggregated-outline-predicate'.

(cherry picked from commit 840be8a7d8e7db8ad7f186678226ac51712724ab)