Add treesit-admin--last-compatible-grammar-for-modes

Add treesit-admin--last-compatible-grammar-for-modes

This function computes the latest compatible grammar version for
each major mode.

* admin/treesit-admin.el (treesit-admin-file-name): Move.
(treesit-admin--compat-template-file-name): New variable.
(treesit-admin--mode-languages): New function.
(treesit-admin--find-latest-compatible-revision): Return a plist
instead.
(treesit-admin--last-compatible-grammar-for-modes): New function.

(cherry picked from commit c9624c21117f7bece4d5a8271888155de5d086a2)