Add treesit-node-top-level

Add treesit-node-top-level

Merge treesit-node-top-level-p and treesit--find-top-level-match into
treesit-node-top-level.

* doc/lispref/parsing.texi (Retrieving Node): Add manual entry for it.
* lisp/progmodes/js.el (js--treesit-imenu-1): Replace
treesit-node-top-level-p with treesit-node-top-level.
* lisp/treesit.el (treesit-node-top-level-p)
(treesit--find-top-level-match): Remove functions.
(treesit-node-top-level): New function.
* lisp/treesit.el (treesit-beginning-of-defun)
(treesit-end-of-defun): Replace treesit--find-top-level-match with
treesit-node-top-level.