Add game_template (#1180)

Adds a template / sample for games built in Flutter, with all the bells and whistles, like ads, in-app purchases, audio, main menu, settings, and so on.

Co-authored-by: Parker Lougheed
Co-authored-by: Shams Zakhour

game_template/lib/src/game_internals/level_state.dart

@@ -0,0 +1,32 @@
+// Copyright 2022, the Flutter project authors. Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'package:flutter/foundation.dart';
+
+/// An extremely silly example of a game state.
+///
+/// Tracks only a single variable, [progress], and calls [onWin] when
+/// the value of [progress] reaches [goal].
+class LevelState extends ChangeNotifier {
+  final VoidCallback onWin;
+
+  final int goal;
+
+  LevelState({required this.onWin, this.goal = 100});
+
+  int _progress = 0;
+
+  int get progress => _progress;
+
+  void setProgress(int value) {
+    _progress = value;
+    notifyListeners();
+  }
+
+  void evaluate() {
+    if (_progress >= goal) {
+      onWin();
+    }
+  }
+}