feat(grfn/xanthous): Add a help panel

Change-Id: I581a37df0a20fa54878da4446007dbe677e057da
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5444
Autosubmit: grfn <grfn@gws.fyi>
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

users/grfn/xanthous/src/Xanthous/Data/App.hs

@@ -16,7 +16,9 @@ import Xanthous.Util.QuickCheck
 
 -- | Enum for "panels" displayed in the game's UI.
 data Panel
-  = -- | A panel displaying the character's inventory
+  = -- | A panel providing help with the game's commands
+    HelpPanel
+  | -- | A panel displaying the character's inventory
     InventoryPanel
   | -- | A panel describing an item in the inventory in detail
     --