bug#58506: Use ".dir-locals.eld" and ".dir-locals-2.eld" when they exist

[Lars Ingebrigtsen]

Stefan Kangas <stefankangas@gmail.com> writes:

> So how about doing something like the below, as a future compatibility
> patch?  Then, in some undefined future version, we can consider creating
> ".dir-locals.eld" files by default, instead.

It sort of makes sense to me, but if we're doing a name change, I'd
rather make it a functional change, too.

It's been pointed out many times that the .dir-locals.el file format is
bad -- it's not extensible, and many things have been proposed for extra
functionality that's impossible to add today.

(For instance -- variables set before the major mode happens, and adding
to list variables.)

So if we introduce .dir-locals.eld, we should take the opportunity to
add a brand new, sensible, extensible syntax, too.