Add options for setting up windows

lib/modules/valhalla.nix

@@ -11,6 +11,7 @@
       ./partition.nix
       ./software.nix
       ./users.nix
+      ./windows.nix
     ];
 
     options = {

lib/modules/windows.nix

@@ -0,0 +1,37 @@
+{ lib, ... }:
+  let
+    inherit (lib)
+      mkOption
+      mkEnableOption
+      mkIf
+      types
+    ;
+
+    winType = types.submodule (
+      { config, ... }: {
+        options = {
+          dualboot = {
+            enable = mkEnableOption "dual boot";
+
+            linuxPercentage = mkOption {
+              type = types.number;
+              description = "The percentage of the disk size reserved for Linux.";
+            };
+          };
+        };
+
+        config = {
+          dualboot.linuxPercentage = mkIf (!config.dualboot.enable) 0;
+        };
+      });
+  in {
+    options = {
+      valhalla = {
+        windows = mkOption {
+          type = winType;
+          description = "The options for setting up Windows.";
+          default = {};
+        };
+      };
+    };
+  }