sddm: add numlock switch

- added numlock on boot switch
- simply add :
services.xserver.displayManager.sddm.autoNumlock = true;
to configuration.nix and sddm will start
with numlock enabled.

nixos/modules/services/x11/display-managers/sddm.nix

@@ -31,6 +31,9 @@ let
     [General]
     HaltCommand=${pkgs.systemd}/bin/systemctl poweroff
     RebootCommand=${pkgs.systemd}/bin/systemctl reboot
+    ${optionalString cfg.autoNumlock ''
+    Numlock=on
+    ''}
 
     [Theme]
     Current=${cfg.theme}
@@ -110,6 +113,14 @@ in
         '';
       };
 
+      autoNumlock = mkOption {
+        type = types.bool;
+        default = false;
+        description = ''
+          Enable numlock at login.
+        '';
+      };
+
       setupScript = mkOption {
         type = types.str;
         default = "";