[Desktop/Plasma] add Plasma 6

Allow both desktops to exist at the same time.
Add "plasma6" environment option.

desktop/plasma/plasma5.nix

@@ -0,0 +1,29 @@
+{ config, pkgs, lib, ... }:
+with lib;
+let
+  cfg = config.aviallon.desktop;
+  generic = import ./generic.nix {
+    kdePackages = pkgs.libsForQt5;
+  };
+in {
+  config = mkIf (cfg.enable && (cfg.environment == "plasma")) {
+    # Enable the Plasma 5 Desktop Environment.
+    services.xserver.desktopManager.plasma5 = {
+      enable = true;
+      runUsingSystemd = true;
+      useQtScaling = true;
+
+      # Removed in: https://github.com/NixOS/nixpkgs/pull/172078
+      # and: https://github.com/NixOS/nixpkgs/pull/221721
+      # Once this (https://invent.kde.org/plasma/powerdevil/-/issues/19) is solved, make PR to add it back (prehaps by default?)
+      # supportDDC = true;
+    };
+
+    environment.systemPackages = generic.commonPackages ++ [
+      pkgs.kio-fuse
+    ];
+
+    # We prefer Plasma Wayland
+    services.displayManager.defaultSession = "plasmawayland";
+  };
+}