feature(sddm): add sddm module for theming and session management

modules/nixos/sddm.nix

@@ -0,0 +1,20 @@
+{ config, lib, pkgs, ... }:
+
+{
+  # enable sddm and use astronaut theme
+  services.displayManager.sddm = {
+    enable = true;
+    wayland.enable = true;
+    autoNumlock = true;
+    enableHidpi = true;
+    theme = "sddm-astronaut-theme";
+    settings = {
+      Theme = {
+        Current = "sddm-astronaut-theme";
+      };
+    };
+    extraPackages = with pkgs; [
+      sddm-astronaut
+    ];
+  };
+}