Introduce a "user" module that can be interpolated everywhere.

Signed-off-by: Mariano Uvalle <juvallegarza@microsoft.com>

modules/common/default.nix

@@ -0,0 +1,5 @@
+{
+  imports = [
+    ./user-config.nix
+  ];
+}

modules/common/user-config.nix

@@ -0,0 +1,24 @@
+{ lib, pkgs, config, ... }:
+
+{
+  options.user = lib.mkOption {
+    type = lib.types.submodule {
+      options = {
+        name = lib.mkOption {
+          type = lib.types.str;
+          description = "Primary username for the system";
+        };
+
+        homeDirectory = lib.mkOption {
+          type = lib.types.str;
+          description = "Home directory path for the user";
+          default =
+            if pkgs.stdenv.isDarwin
+            then "/Users/${config.user.name}"
+            else "/home/${config.user.name}";
+        };
+      };
+    };
+    description = "User configuration options";
+  };
+}