chore(nix): add flake

[TornaxO7]

Adds a dev shell for nix users.

[mdecimus]

I'm not familiar with Nix, why is a flake needed when dependencies are handled by cargo?

[TornaxO7]

Oh damn, I completely forgot this one....

why is a flake needed when dependencies are handled by cargo?

This is just an addition for nix-users like me.
The flake is there to help nix-users to load a shell which setups everything (in this case: Install the correct rust toolchain) and by using direnv this happens by simply navigating into the project.

In my case, by having this PR merged I just need to cd into the project and every dev tool (rust-analyzer, rust compiler, cargo, etc.) get prepared for automatically without doing anything and I can immediately start working.