I'd like to remove `dev-env`. It's served us well, but its original
ambitions were to go way beyond a simple `nix-shell` equivalent, and now
that it's all we're using it for it doesn't really add much anymore.
Using a standard nix-shell setup would reduce the complexity of this
repo and make it easier for other developers to jump in. It would also
somewhat reduce the dev-env verbosity, which is a minor annoyance.
This is, however, a big change, and I don't think trying to do it in one
go is a great idea. So instead I'm setting a foundation in this PR and
plan to move step by step over several follow-up PRs. In this one I just
add a small default nix-shell configuration and add it to `.envrc`. In
follow-up PRs, I'll be moving paclages over from the dev-env
configuration to the nix shell, up to the point where dev-env is just an
empty shell that we can easily remove.
This PR also serves as a not-so-implicit way of gathering support for
this plan.
