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/constants.dart

@@ -0,0 +1,3 @@
+class Constants {
+  static const double tileSize = 50;
+}