Further tweak tree-sitter fontification heuristics

So it turns out the slowness observed in bug#59415 is not due to the
size, but the strangely tall tree.  Adjust the heuristic to DTRT:
don't enable the heuristic by default or when buffer is large, enable
when query is abnormally slow.  We could do some clever thing that
calibrates a base reading for the query time instead of using a
hard-coded value, but it doesn't seem necessary.

* lisp/treesit.el (treesit--font-lock-fast-mode): New variable.
(treesit-font-lock-fontify-region): Don't activate heuristic by
default (reasons in comments).  Measure the query time and activate
the fast mode if query time is long.

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