feat: SpawnComponent (#2709)

This PR introduces the `SpawnComponent`, which randomly spawns
components within a set area.

examples/lib/stories/components/spawn_component_example.dart

@@ -0,0 +1,60 @@
+import 'package:examples/commons/ember.dart';
+import 'package:flame/components.dart';
+import 'package:flame/experimental.dart';
+import 'package:flame/extensions.dart';
+import 'package:flame/game.dart';
+import 'package:flame/input.dart';
+import 'package:flame/math.dart';
+
+class SpawnComponentExample extends FlameGame with TapDetector {
+  static String description =
+      'Tap on the screen to start spawning Embers within different shapes.';
+
+  @override
+  void onTapDown(TapDownInfo info) {
+    final shapeType = Shapes.values.random();
+    final Shape shape;
+    final position = info.eventPosition.game;
+    switch (shapeType) {
+      case Shapes.rectangle:
+        shape = Rectangle.fromCenter(
+          center: info.eventPosition.game,
+          size: Vector2.all(200),
+        );
+      case Shapes.circle:
+        shape = Circle(info.eventPosition.game, 150);
+      case Shapes.polygon:
+        shape = Polygon(
+          [
+            Vector2(-1.0, 0.0),
+            Vector2(-0.8, 0.6),
+            Vector2(0.0, 1.0),
+            Vector2(0.6, 0.9),
+            Vector2(1.0, 0.0),
+            Vector2(0.3, -0.2),
+            Vector2(0.0, -1.0),
+            Vector2(-0.8, -0.5),
+          ].map((vertex) {
+            return vertex
+              ..scale(200)
+              ..add(position);
+          }).toList(),
+        );
+    }
+
+    add(
+      SpawnComponent(
+        factory: (_) => Ember(),
+        period: 0.5,
+        area: shape,
+        within: randomFallback.nextBool(),
+      ),
+    );
+  }
+}
+
+enum Shapes {
+  rectangle,
+  circle,
+  polygon,
+}