refactor(modules/system): move system config to a module

modules/nixos/system/default.nix

@@ -0,0 +1,16 @@
+{ config, lib, ...}:
+{
+  imports = [
+    ./users.nix
+    ./localisation.nix
+    ./nix-config.nix
+  ];
+
+  options.system = {
+   mainUser = lib.pantheon.mkStrOption;
+  };
+
+  config = {
+    system.stateVersion = "25.05"; # Did you read the comment?
+  };
+}

modules/nixos/system/localisation.nix

@@ -0,0 +1,9 @@
+{ config, lib, ... }:
+{
+  config = lib.mkMerge [
+    {
+	  time.timeZone = "Asia/Singapore";
+	  i18n.defaultLocale = "en_US.UTF-8";
+    }
+  ];
+}

modules/nixos/system/nix-config.nix

@@ -0,0 +1,6 @@
+{ config, lib, ... }:
+{
+  config = {
+    nix.settings.experimental-features = ["nix-command" "flakes"];
+  };
+}

modules/nixos/system/users.nix

@@ -0,0 +1,18 @@
+{ config, lib, pkgs, ... }:
+{
+  config = lib.mkMerge [
+    {
+      users.users."${config.system.mainUser}" = {
+        isNormalUser = true;
+        initialPassword = "1";
+        extraGroups = [ 
+	  "wheel" 
+	];
+        packages = with pkgs; [
+          git
+          neovim
+        ];
+      };
+    }
+  ];
+}