fish shell versioning

[Paolo Vincenzo Olivo <vins%NetBSD.org@localhost>]

Hi,
Currently we have 2 packages for fish shell:

* shells/fish: the original 3.x branch, now unmaintained upstream.
* shells/fish4: the 4.x branch, which is a complete rewrite in Rust.

We may alternatively:

1. keep things as they are now.
2. update shells/fish with shells/fish4 and move the 3.x branch to
   shells/fish3. In case, decide whether this should be done before or
   after the freeze.

In any case I think it's advisable to keep fish 3.x in repo, since
having a shell depend on Rust makes it unavailable on several "minor"
platforms.

--
PVO