I think I have a clear enough idea of how support for Editorconfig
should be integrated into Emacs, in terms of "how to hook it to the
right places".  There are still pending questions about how to adapt
the code of the `editorconfig` package, but in the mean time: here's
a first step to make sure Emacs offers the right hooks.

Find below two patch which do that.

The first patch slightly extends `auto-coding-functions` so that it can
use the file name to decide which coding system to use.  This is
a fairly simple change: this clearly the right place to hook ourselves
into.  I make the change by adding a dynamically scoped
`auto-coding-file-name` variable.  The resulting API would be cleaner if
we instead passed the filename as an additional argument, which we could
do easily (even with some amount of backward compatibility), but the
change proposed has the advantage of being 100% backward compatible.
The cleaner API would replace:

    (let ((auto-coding-file-name filename))
      (funcall (pop funcs) size))

with

    (let ((fun (pop funcs)))
      (condition-case nil
          (funcall fun size filename)
        (wrong-number-of-arguments (funcall fun size))))

In the long run the cleaner API is probably preferable, but it does come
with a slight risk of backward incompatibility.

The second patch adds a new hook
`hack-dir-local-get-variables-functions`.  This one is not as "obvious":
there are other places we could hook ourselves into.  E.g. it's tempting
to hook deeper into the dir-locals machinery so as to reuse its caching
mechanism.  In the end I decided not to do that because the dir-locals
machinery is not prepared to accept settings coming from different
levels of the directory tree, so that trying to shoehorn Editorconfig
into it would be a bit messy and/or unreliable.
The downside is that this hook forces its users to take care of their
own caching.  But since the current examples all need to do their
caching a bit differently, I think this is OK.

There's one other design choice I made:
`hack-dir-local-get-variables-functions` returns elements of the form
(DIR . ALIST) where DIR is used to obey
`safe-local-variable-directories' as well as to provide a bit more info
to the user when prompting for confirmation in cache of risky settings.
But that does not provide any actual info about the actual source of the
ALIST, so the message we emit when prompting the user can't tell the
user whether the settings come from a `.dir-locals.el` or
something else.  This is not a new problem: the current code already
can't tell the user if the settings come from a `.dir-locals.el`, or
a "project class" or a `.dir-locals-2.el`.  So, apparently, this is
a problem we have not deemed worthwhile to fix, so I left it
like unsolved.

I'm hoping this can get into Emacs-30.
Comments/objections?


        Stefan