Use tree-sitter for font-lock's syntactic function (optionally)

Use tree-sitter for font-lock's syntactic function (optionally)

Before this change, we rebind font-lock-fontify-region-function and
call font-lock's function in tree-sitter's function, and the order of
fontification is tree-sitter, font-lock syntax, font-lock regexp.

Now we make font-lock's syntax function customizable and replace it
with tree-sitter's function, and the order of fontification is
tree-sitter, font-lock regexp.

* doc/lispref/modes.texi (Parser-based Font Lock): Reflect the change
in manual.
* lisp/font-lock.el (font-lock-fontify-syntactically-function): New
varaible.
(font-lock-default-fontify-region): Call
font-lock-fontify-syntactically-function rather.
(font-lock-fontify-syntactically-region): Rename to
font-lock-default-fontify-syntactically
(font-lock-default-fontify-syntactically): Rename to this.

* lisp/treesit.el (treesit-font-lock-fontify-region): Don't call
font-lock functions.
(treesit-font-lock-enable): Remove this function.  It is not used even
before this change.
(treesit-major-mode-setup): Instead of binding
font-lock-fontify-region-function, now bind to
font-lock-fontify-syntactically-function.  And we can let font-lock do
it's thing.