Fix tree-sitter fontification heuristic

Fix tree-sitter fontification heuristic

Previously applied heuristic 2 sometimes invalidates heuristic 1, add
a guard so it doesn't.

The new function is just for clearity of the code and has nothing to
do with the change itself.

* lisp/treesit.el (treesit--node-length): New function
(treesit-font-lock-fontify-region): Use the new function.  Only do
heuristic 2 when the node is large.