feat(gs/xanthous): Default to the current save file

When saving, default to the save file that was loaded for the game if
any. To support this, this also makes text prompts support a default,
which will be used if no value is input.

Change-Id: I72a826499d6e987b939e3465a2d29167e53416be
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3801
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

users/grfn/xanthous/app/Main.hs

@@ -113,9 +113,9 @@ newGame rparams = do
 
 loadGame :: FilePath -> IO ()
 loadGame saveFile = do
-  gameState <- maybe (die "Invalid save file!") pure
-              =<< Game.loadGame . fromStrict <$> readFile @IO saveFile
-  gameState `deepseq` runGame LoadGame gameState
+  gameState <- maybe (die "Invalid save file!") pure . Game.loadGame  . fromStrict
+              =<< readFile @IO saveFile
+  gameState `deepseq` runGame (LoadGame saveFile) gameState
 
 runGame :: RunType -> Game.GameState -> IO ()
 runGame rt gameState = do