Add treesit-defun-name and friends

Add treesit-defun-name and friends

1. We now have treesit-defun-name, powered by
treesit-defun-name-function.
2. We now have treesit-add-log-current-defun, which powers
add-log-current-defun.
3. c-ts-mode updates its code to take advantage of these new features.
4. Manual updates.

* doc/lispref/parsing.texi (Tree-sitter major modes): Add manual for
new functions.
* lisp/progmodes/c-ts-mode.el (c-ts-mode--defun-name): New function.
(c-ts-mode--imenu-1): Extract out into c-ts-mode--defun-name.
(c-ts-base-mode): Setup treesit-defun-name-function.
* lisp/treesit.el (treesit-defun-name-function)
(treesit-add-log-defun-delimiter): New variables.
(treesit-defun-at-point)
(treesit-defun-name): New functions.
(treesit-major-mode-setup): Setup add-log-current-defun-function.