Further generalize treesit-defun functions

Further generalize treesit-defun functions

Two new functions, treesit-beginning/end-of-thing.  And
treesit-thing-at-point's signature changes.

* lisp/treesit.el (treesit-block-type-regexp): New variable.
(treesit-beginning-of-thing)
(treesit-end-of-thing): Generalized from
treesit-beginning/end-of-defun.
(treesit-beginning-of-defun)
(treesit-end-of-defun): Use the new functions.
(treesit-thing-at-point): Accept PATTERN rather than REGEXP and PRED.
(treesit-defun-at-point): Adjust for the new signature of
treesit-thing-at-point.