Add tree-sitter helper functions for Imenu

Add tree-sitter helper functions for Imenu

We didn't add an integration for Imenu because we aren't sure what
should it look like.  Now we have a pretty good idea.  All the major
modes copy-paste the two Imenu functions and tweaks them in a standard
way.  With the addition of treesit-defun-type-regexp and
treesit-defun-name-function, now is a good time to standardize Imenu
integration.

In the next commit we update all the major modes to use this
integration.

* doc/lispref/modes.texi (Imenu): Add manual.
* doc/lispref/parsing.texi (Tree-sitter major modes): Update manual.
* lisp/treesit.el (treesit-simple-imenu-settings): New varaible.
(treesit--simple-imenu-1)
(treesit-simple-imenu): New functions.
(treesit-major-mode-setup): Setup Imenu.