postMessage method
Post message to unity from flutter. This method takes in a string message.
The gameObject must match the name of an actual unity game object in a scene at runtime, and the methodName,
must exist in a MonoDevelop class and also exposed as a method. message is an parameter taken by the method
postMessage("GameManager", "openScene", "ThirdScene")
Implementation
Future<void>? postMessage(String gameObject, methodName, message) {
throw UnimplementedError('postMessage() has not been implemented.');
}