Implement Home Manager template

[erikreinert]

Changes

• adds new home-manager template for non-NixOS and nix-darwin systems.

[spikespaz]

Aside from new Nix users thinking they need flake-parts after reading this code, LGTM.

{
    inputs = {
        systems.url = "github:nix-systems/default";
        # ...
    };

    outputs = inputs@{systems, nixpkgs, ...}: let
        inherit (nixpkgs) lib;
        eachSystem = lib.genAttrs (import systems);
        pkgsFor = eachSystem (system:
            import nixpkgs {
                localSystem.system = system;
            }
        );
    in {
        packages = eachSystem (system: let pkgs = pkgsFor.${system}; in {
            # ...
        });
        # ...
    }
}

[erikreinert]

I think it would be worth while to add in the README somewhere about that we use flake-parts as a guide and is not always required. This would be helpful to guide people away if they need to which I'm down to promote.