Make it harder to misactivate tree-sitter font-lock fast mode

Make it harder to misactivate tree-sitter font-lock fast mode

This has been brought up in bug#60691 and bug#60223.  I proposed a fix
by testing the size of the tree rather than measuring the query time.
But after some thought, I fear that just looking at the size will give
us false-negatives.  So I kept the time-based activation, just added a
grace count to reduce false-positives.

* lisp/treesit.el:
(treesit--font-lock-fast-mode-grace-count): New variable.
(treesit--font-lock-notifier): Only activate fast mode after 5
offenses.