Update to (somewhat?) dendritic pattern

Heavily inspired by https://github.com/Doc-Steve/dendritic-design-with-flake-parts.

> Atomic commits? Never heard of them!

modules/factory/user.nix

@@ -0,0 +1,30 @@
+{ self, ... }: {
+  config.flake.factory.user = username: isAdmin: {
+    nixos."${username}" =
+      { lib
+      , pkgs
+      , ...
+      }: {
+        users.users."${username}" = {
+          isNormalUser = true;
+          home = "/home/${username}";
+          extraGroups = lib.optionals isAdmin [ "wheel" ];
+          shell = pkgs.zsh;
+        };
+
+        programs.zsh.enable = true;
+        # required for zsh completion
+        environment.pathsToLink = [ "/share/zsh" ];
+
+        home-manager.users."${username}" = {
+          imports = [
+            self.modules.homeManager."${username}"
+          ];
+        };
+      };
+
+    homeManager."${username}" = {
+      home.username = "${username}";
+    };
+  };
+}