Partial support for DEFUN in c-ts-mode (bug#62825)

Partial support for DEFUN in c-ts-mode (bug#62825)

The DEFUN is hard to incorporate because it's made of two nodes rather
than one, and most tree-sitter functionalities assume a defun is one
node.  I fixed the indent-defun and add-log functionality, but
beginning/end-of-defun and imenu still don't recognize DEFUN.

* lisp/progmodes/c-ts-mode.el (c-ts-mode-emacs-devel): New variable.
(c-ts-mode--defun-name): Support DEFUN.
(c-ts-mode--defun-valid-p): Support DEFUN.
(c-ts-mode--emacs-defun-p)
(c-ts-mode--emacs-defun-at-point): New functions.
(c-ts-mode-indent-defun): Use c-ts-mode--emacs-defun-at-point.
(c-ts-mode--emacs-current-defun-name): New function.
(c-ts-mode, c++-ts-mode): Optionally setup custom defun-name function.