refactor: split code into modules

modules/base/nh.nix

@@ -0,0 +1,16 @@
+{
+  config,
+  username,
+  ...
+}: {
+  programs.nh = {
+    enable = true;
+
+    clean = {
+      enable = !config.nix.gc.automatic;
+      dates = "weekly";
+    };
+
+    flake = "/home/${username}/Projects/nixos-config";
+  };
+}