Fix treesit--things-around (bug#60355)

Fix treesit--things-around (bug#60355)

Current implementation of treesit--things-around only searches forward
for REGEXP and go up the tree until it finds a valid thing, if nothing
matches it gives up.  This makes it sometimes miss defuns.  The new
implementation tries multiple times (of search forward + go up) until
it exhausts all possible defun nodes.

* lisp/treesit.el (treesit--things-around): New implementation.
(treesit--navigate-defun): Refactor to use treesit-node-top-level to
simplify code, and add some guards in the predicate function.
* test/src/treesit-tests.el:
(treesit--ert-defun-navigation-elixir-program): New variable.
(treesit-defun-navigation-nested-4): New test.