add isAsyncIterable

we could consider later adding support for $$asyncIterable similar to upstream graphql-js, but this would have to be across the entire codebase, which currently supports only Symbol.asyncIterator.

Author: yaacovCR

packages/utils/src/index.ts

@@ -48,3 +48,4 @@ export * from './observableToAsyncIterable';
 export * from './visitResult';
 export * from './getArgumentValues';
 export * from './valueMatchesCriteria';
+export * from './isAsyncIterable';

packages/utils/src/isAsyncIterable.ts

@@ -0,0 +1,3 @@
+export function isAsyncIterable<T>(value: any): value is AsyncIterable<T> {
+  return Symbol.asyncIterator in value;
+}