Make tree-sitter thing feature public and remove obsolete functions

Make tree-sitter thing feature public and remove obsolete functions

* lisp/treesit.el (treesit--things-around): Remove function.
(treesit-forward-sexp):
(treesit-beginning-of-thing):
(treesit-end-of-thing):
(treesit-navigate-thing):
(treesit-thing-at-point):
(treesit-outline-search): Use public version of thing-functions.
(treesit--thing-prev):
(treesit--thing-next):
(treesit--thing-at):
(treesit--navigate-thing): Make public.

* test/src/treesit-tests.el (treesit--ert-test-defun-navigation): Use
public version of thing-functions.