Add coins to game

This adds a new asset, a new component and some new game mechanics.
Coins will appear every one to two minutes and can be collected by
tapping them.

lib/components/coin.dart

@@ -0,0 +1,75 @@
+import 'dart:ui';
+
+import 'package:flame/anchor.dart';
+import 'package:flame/components/component.dart';
+import 'package:flame/sprite.dart';
+import 'package:flame/time.dart';
+import 'package:flutter/widgets.dart';
+import 'package:vector_math/vector_math_64.dart';
+
+import '../storage.dart';
+
+class Coin extends PositionComponent {
+  static const flightTime = 0.8;
+
+  final sprite = Sprite('coin.png');
+  final Vector2 position;
+  final Vector2 target;
+
+  Timer killTimer;
+  bool shouldDestroy = false;
+
+  bool flying = false;
+  double flight = 0.0;
+  
+  Coin(this.position, this.target, void Function() onDestroy) {
+    x = position.x;
+    y = position.y;
+    width = 24.0;
+    height = 24.0;
+    anchor = Anchor.center;
+
+    killTimer = Timer(2.5, callback: () {
+      onDestroy();
+      shouldDestroy = true;
+    });
+
+    killTimer.start();
+  }
+
+  void tap() {
+    killTimer.stop();
+    flying = true;
+    storage.addCoins(1);
+  }
+
+  @override
+  void render(Canvas c) {
+    prepareCanvas(c);
+    sprite.render(c, width: width, height: height);
+  }
+
+  @override
+  void update(double t) {
+    killTimer.update(t);
+
+    if (flying) {
+      flight += t / flightTime;
+
+      if (flight > 1.0) {
+        flight = 1.0;
+        flying = false;
+        shouldDestroy = true;
+      }
+
+      x = lerpDouble(position.x, target.x, Curves.easeIn.transform(flight));
+      y = lerpDouble(position.y, target.y, Curves.easeInBack.transform(flight));
+    }
+  }
+
+  @override
+  bool destroy() => shouldDestroy;
+
+  @override
+  int priority() => 25000;
+}