Add an argument OVERRIDE to tree-sitter font-lock functions

Add an argument OVERRIDE to tree-sitter font-lock functions

* doc/lispref/modes.texi (Parser-based Font Lock): Reflect this change
in manual.
* lisp/progmodes/js.el (js--fontify-template-string): Add _OVERRIDE
argument.
* lisp/progmodes/python.el (python--treesit-fontify-string): Add
_OVERRIDE argument.
* lisp/treesit.el (treesit-font-lock-rules): Update docstring.
(treesit-fontify-with-override): New function.
(treesit-font-lock-fontify-region): Extract out into
treesit-fontify-with-override.