Make tree-sitter fontification automatically update

Make tree-sitter fontification automatically update

Now nodes that are affected and changed during re-parse will be
correctly refontified.  Nodes can change even if it corresponding text
wasn't edited: additional text can complete the parse tree and resolve
error nodes, for example.

* lisp/progmodes/python.el (python-mode): Create Python parser before
calling treesit-major-mode-setup.
* lisp/treesit.el (treesit--font-lock-notifier): New function.
(treesit-major-mode-setup): Register fontifier with every existing
parser.