Move nix and snap packages to a different repo

[b1ek]

we have nix and snap packages hosted in this repo, and even CI pipelines for them (nix is pending on #385)

i don't like it here (in this repo) because of #294

[Mte90]

I agree we have also other tasks about releases, ideally when it is time for a new release in this other repo we just create a new release and a CI generates all of them.

[mks-h]

Just for the reference — we have not only the nix package, but also the development environment defined in the same nix flake file. IMHO we don't really benefit from it, as we don't have any notable dependencies that wouldn't be installed on a typical Rust developer's system. So it should be fine to remove.

[Thesola10]

Keeping the Nix flake in this repository makes it easier to install and integrate into other projects using Nix. I'm currently working on something that leverages this, and seeing as 0.4.0 hasn't made it to Nixpkgs yet, I'm using this repository itself as an input package.

When it comes to Nix specifically, I think the build files should be kept in-repo, at the very least the flake.nix should.

Some projects use a nix/ directory to place other Nix source files when things get too heavy, maybe this could apply here too?