refactor: make all GameOptions optional with sensible defaults

- cols defaults to 30 (was required)
- rows defaults to 20 (was required)
- timeLimit and startingLevel remain optional
- Can now call useGame() with no arguments
- Example: useGame(), useGame({}), useGame({ startingLevel: 5 })

example/src/App.tsx

@@ -3,7 +3,7 @@ import "@vimazing/vim-snake/game.css";
 import { useKeyBindings } from "./useKeyBindings";
 
 function App() {
-  const { containerRef, gameStatus, scoreManager, keyLog, score, level } = useGame({ cols: 30, rows: 20 }, useKeyBindings);
+  const { containerRef, gameStatus, scoreManager, keyLog, score, level } = useGame({}, useKeyBindings);
 
   const formatTime = (ms: number) => {
     const seconds = Math.floor(ms / 1000);