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

[Gregory Heytings]

> > I didn't yet have time to try the branch, but what you say begs a 
> > question: does the branch attempt to solve the problems of JSON files 
> > only, or does it attempt to solve a more general problem?
>
> There is nothing json-specific in the branch. All the js/json stuff is 
> on master already.

That doesn't answer Eli's question, AFAIU.  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.