TS: use proper type for async generator (#3009)

Author: saihaj

integrationTests/ts/tsconfig.json

@@ -1,7 +1,7 @@
 {
   "compilerOptions": {
     "module": "commonjs",
-    "lib": ["es6", "esnext.asynciterable"],
+    "lib": ["es2019", "es2020.string"],
     "strict": true,
     "noEmit": true,
     "types": []

src/subscription/mapAsyncIterator.d.ts

@@ -5,7 +5,7 @@ import { PromiseOrValue } from '../jsutils/PromiseOrValue';
  * which produces values mapped via calling the callback function.
  */
 export function mapAsyncIterator<T, U>(
-  iterable: AsyncIterable<T>,
+  iterable: AsyncIterable<T> | AsyncGenerator<T, void, void>,
   callback: (arg: T) => PromiseOrValue<U>,
   rejectCallback?: (arg: any) => PromiseOrValue<U>,
-): any; // TS_SPECIFIC: AsyncGenerator requires [email protected]
+): AsyncGenerator<U, void, void>;

src/subscription/subscribe.d.ts

@@ -39,7 +39,7 @@ export interface SubscriptionArgs {
  */
 export function subscribe(
   args: SubscriptionArgs,
-): Promise<AsyncIterableIterator<ExecutionResult> | ExecutionResult>;
+): Promise<AsyncGenerator<ExecutionResult, void, void> | ExecutionResult>;
 
 /**
  * Implements the "CreateSourceEventStream" algorithm described in the