sway: Switch default terminal to alacritty

While st is awesome, supports ligatures and consume just around 10MB of
memory running it requires having to run Xwayland. For now, switch to
alacritty so we need not run Xwayland or any X clients.

If we find a better terminal later on we can switch to that.

sway/.config/sway/config

@@ -12,7 +12,7 @@ set $down  j
 set $up    k
 set $right l
 # Your preferred terminal emulator
-set $term st
+set $term alacritty
 # Your preferred application launcher
 # Note: pass the final command to swaymsg so that the resulting window can be opened
 # on the original workspace that the command was run on.