Allow treesit-simple-indent's rule to be a single function

Allow treesit-simple-indent's rule to be a single function

* lisp/treesit.el (treesit-simple-indent-rules): Allow the rule
to be a single function.  Also replace cl-loop with dolist plus
catch-throw.
(treesit--indent-rules-optimize): Handle the case when a rule is
a function.
* doc/lispref/modes.texi (Parser-based Indentation): Update
manuel.

(cherry picked from commit 1e44c63fcadeb89a9a9c9208f221970e6fcc774c)