Add dwm window manager with some initial patches

- Set Mod4 as the mod key and Alacritty as the termcmd.
- Add a patch from suckless.org to hide any vacant tags.
  - https://dwm.suckless.org/patches/hide_vacant_tags

nix/hosts/t490/services/default.nix

@@ -1,11 +1,10 @@
-{ ... }:
-
 {
   imports = [
+    ./dwm.nix
     ./interception-tools.nix
     ./sound.nix
-    ./xserver.nix
     ./udev.nix
+    ./xserver.nix
   ];
 
   services = {