Generalize treesit-defun functions to "things"

Generalize treesit-defun functions to "things"

Change the "defun" in some functions (e.g. treesit--defuns-around) to
"thing".  Add a function treesit-thing-at-point.

* lisp/treesit.el (treesit--thing-unpack-pattern): New subroutine.
(treesit-beginning-of-defun)
(treesit-end-of-defun): Use new function treesit--navigate-thing.
(treesit--defuns-around): Generalize into treesit--thing-around.
(treesit--top-level-defun): Generalize into treesit--top-level-thing.
(treesit--navigate-defun): Generalize into treesit--navigate-thing.
(treesit-thing-at-point): Generalized from treesit-defun-at-point.
(treesit-defun-at-point): Use treesit-thing-at-point to do tht work.