Template to add a new service

A `just` command that takes the service name as an argument and creates `nix/.nix` and `nix/_test.nix` with a `hello-world` service example.

+ Maybe also add the files in the respective lists of `nix/default.nix` and `test/flake.nix`?

+ Update README to add the service

+ Comments specifying to take inspiration from either the nixos module or devenv module if either of them already implement the service.