GREEN: save score state and add knocked pins on each roll

lib/bowling_game.dart

@@ -1,7 +1,11 @@
 class BowlingGame {
-  void roll(int pins) {}
+  int _score = 0;
+
+  void roll(int pins) {
+    _score += pins;
+  }
 
   int score() {
-    return 0;
+    return _score;
   }
 }