Add a formatter, which can be used by running "nix fmt"

Use nixpkgs-fmt because what it does closely aligns to this repos
already established conventions and it doesn't do weird stuff.

Also see:
https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-fmt.html
https://github.com/nix-community/nixpkgs-fmt

flake.nix

@@ -220,5 +220,7 @@
         format = "proxmox";
       };
     };
+    
+    formatter.x86_64-linux = nixpkgs.legacyPackages.x86_64-linux.nixpkgs-fmt;
   };
 }