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

patches/dwm/change-mod-key.patch

@@ -0,0 +1,13 @@
+diff --git a/config.def.h b/config.def.h
+index 9efa774..4412cb1 100644
+--- a/config.def.h
++++ b/config.def.h
+@@ -45,7 +45,7 @@ static const Layout layouts[] = {
+ };
+ 
+ /* key definitions */
+-#define MODKEY Mod1Mask
++#define MODKEY Mod4Mask
+ #define TAGKEYS(KEY,TAG) \
+ 	{ MODKEY,                       KEY,      view,           {.ui = 1 << TAG} }, \
+ 	{ MODKEY|ControlMask,           KEY,      toggleview,     {.ui = 1 << TAG} }, \