refactor: renamed egg to extra

lib/game/components/ball.dart

@@ -15,8 +15,10 @@ enum BallType {
   /// the [BottomWall].
   normal,
 
-  /// A [Ball] was spawned by Dash in the [FlutterForest].
-  egg,
+  /// A [Ball] that does not alter [GameState.balls].
+  ///
+  /// For example, a [Ball] spawned by Dash in the [FlutterForest].
+  extra,
 }
 
 /// {@template ball_blueprint}

lib/game/components/flutter_forest.dart

@@ -35,7 +35,7 @@ class FlutterForest extends Component
     gameRef.addFromBlueprint(
       BallBlueprint(
         position: Vector2(17.2, 52.7),
-        type: BallType.egg,
+        type: BallType.extra,
       ),
     );
   }