feat: FlameIsolate - a neat way of handling threads (#1909)

Adding a bridge library for integral_isolates adding support for components to run CPU intensive code with a function similar to Flutter's compute function, but with a long lived isolate. Lifecycle is handled by the game loop, where the isolate would live between onMount and onRemove.

packages/flame_isolate/example/lib/terrain/terrain.dart

@@ -0,0 +1,5 @@
+import 'package:flame/components.dart';
+
+mixin Terrain on PositionComponent {
+  double get difficulty;
+}