Add debug command to reveal the game

Add a (debug) command to reveal all tiles on the game regardless of the
character's vision, which'll make it easier to debug creature's behavior
while they're not visible.

src/Xanthous/Game/Arbitrary.hs

@@ -24,4 +24,5 @@ instance Arbitrary GameState where
     _revealedPositions <- fmap setFromList . sublistOf $ EntityMap.positions _entities
     _randomGen <- mkStdGen <$> arbitrary
     let _promptState = NoPrompt -- TODO
+    _debugState <- arbitrary
     pure $ GameState {..}