some refactoring and modularizing locale

modules/nixos/services/session/xserver/default.nix

@@ -0,0 +1,29 @@
+{
+  config,
+  lib,
+  ...
+}: let
+  inherit (lib) mkOption types;
+  cfg = config.nixos.services.session.xserver;
+in {
+  options = {
+    nixos.services.session.xserver = {
+      videoDrivers = mkOption {
+        type = types.listOf (types.enum ["amdgpu" "nvidia"]);
+        default = ["amdgpu"];
+        description = "The names of the video drivers the configuration supports";
+      };
+      xkbLayout = mkOption {
+        type = types.str;
+        default = "se";
+        description = "X keyboard layout, or multiple keyboard layouts separated by commas.";
+      };
+    };
+  };
+  config = {
+    services.xserver = {
+      videoDrivers = cfg.videoDrivers;
+      xkb.layout = cfg.xkbLayout;
+    };
+  };
+}