Add Nix package

Found this recently and it's basically exactly what I've been wishing I had. If I end up using git issue, I'm certain to make a nix package for it, but would you like it upstreamed here?

When I tried to run shellcheck, I got a lot of warnings that I thought had been suppressed, but while the descriptions are identical the error codes were not. I haven't delved too deeply into it, but I suspect we could be running different versions of shellcheck.

A nix flake would effectively pin dependencies and specific versions for development and install (well, for nix users anyway).