feat: add initial NixOS flake (WIP)

[dr460nf1r3]

This PR adds a NixOS flake, which provides the eruption package. Flakes basically allow anyone to add this repository as source for a system configuration/package, therefore this is a first step of making it available to NixOS users.

Done:

• Building eruption package via rustPlatform.buildRustPackage

To-do:

• Add Systemd units via Nix functions
• Add the package to either Nixpkgs or Chaotic Nyx
• Fix Nearsk build (currently errors out, perfect for adding a bleeding edge eruption_git package as flake input though)