Install nh and friends, modularize nixos conf.

modules/nixos/plasma.nix

@@ -0,0 +1,21 @@
+{
+  pkgs,
+  config,
+  lib,
+  ...
+}: {
+  options = {
+    crony.plasma.enable = lib.mkEnableOption "enable plasma";
+  };
+
+  config = lib.mkIf config.crony.plasma.enable {
+    # Enable the KDE Plasma Desktop Environment.
+    services.displayManager.sddm.enable = true;
+    services.desktopManager.plasma6.enable = true;
+    environment.plasma6.excludePackages = with pkgs.kdePackages; [
+      plasma-browser-integration
+      konsole
+      oxygen
+    ];
+  };
+}