Add the initial devenv

[farcaller]

This adds a basic nix-themed dev shell for lemmy development flow (and is otherwise a noop if you don't have nix in your OS).

It can build the package, however the nix build itself is broken (due to submodules). It's not a critical feature, though.

Also adds the postgres into the shell, to support straight-on lemmy development.

The rust toolchain is frozen at stable, which is enough to build lemmy at the time of writing.

[farcaller]

Not sure about the CI failure as it's in the file I didn't touch.

[Nutomic]

Would be good if someone who is familiar with nix could review this.

[RaitoBezarius]

This adds a ton of nixOS specific stuff into the code base, and none of the main developers use it. Wouldn't these additions be better suited to a fork, or a new project that handles nixOS specifically?

We already have Lemmy packaged in NixOS and Nixpkgs, this is a development environment, while a development environment can live outside of the repo, it may defeat the purpose of this PR to do so, I imagine.

I personally don't commit my Nix development environments to the upstreams whom I work with except they are already using Nix mostly.

[farcaller]

That's fair. My reasoning was that it was easier to have specific rust dependencies and to have a sidecar postgres in the shell (given lemmy doesn't work without postgres at all), but it's fair that you can just use docker compose for it all.