Add tree-sitter font-lock settings helper function/macro

Add tree-sitter font-lock settings helper function/macro

1. Add treesit-font-lock-rules that helps with settings
treesit-font-lock-settings.
2. Remove treesit-font-lock-defaults and with it, decoration levels.

Now major modes should set treesit-font-lock-settings with the output
of treesit-font-lock-rules rather than setting
treesit-font-lock-defaults.

* lisp/treesit.el (treesit-font-lock-settings): Update docstring.
(treesit-font-lock-rules): New function.
(treesit-font-lock-defaults): Remove variable.
(treesit-font-lock-enable): Remove code that interacts
treesit-font-lock-defaults.
* doc/lispref/modes.texi: Update manual for
treesit-font-lock-settings, treesit-font-lock-rules,
treesit-font-lock-defaults.