bug#56682: Fix the long lines font locking related slowdowns

[Stefan Monnier]

> used, for some reason) jit-lock-defer feature: if you set
> jit-lock-defer-time to a large enough value, M-> followed by C-p will
> not be as slow as they are now, I think.

I think currently, with such a setting, in Gregory's example, the 50s of
`syntax-ppss` will still bite the users because they're uninterruptible,
although if the users happen to leave Emacs idle at the right time they
may not notice.

We could/should make sure the 50s are done "in the background" and can
be interrupted at any time.


        Stefan