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

[Eli Zaretskii]

> Date: Fri, 12 Aug 2022 21:41:36 +0000
> From: Gregory Heytings <gregory@heytings.org>
> cc: Eli Zaretskii <eliz@gnu.org>, 56682@debbugs.gnu.org, 
>     monnier@iro.umontreal.ca
> 
> The branch demonstrates that, with the Javascript/JSON improvements
> you installed recently on master, editing the 20 MB dictionary.json
> file without using locked narrowing around fontification-functions
> is reasonably efficient.

Is 20MB single-line JSON file fairly representative of such files out
there?  Do we know of JSON file with much longer lines, for example
50MB or 100MB?

If 20MB is reasonably representative of the long-line JSON, then
perhaps at least json-mode could consider resetting
long-line-threshold to nil.  But if there are significantly larger
files out there, we should first assess the performance in those
cases.