Implement in-memory orchestration backend for testing

Author: YunchuWang

packages/durabletask-js/src/index.ts

@@ -34,3 +34,6 @@ export { TOrchestrator } from "./types/orchestrator.type";
 export { TActivity } from "./types/activity.type";
 export { TInput } from "./types/input.type";
 export { TOutput } from "./types/output.type";
+
+// Testing utilities
+export { InMemoryOrchestrationBackend, TestOrchestrationClient, TestOrchestrationWorker } from "./testing";