feat(subscriptions): export getSubscriptionEventSource

Author: Urigo

src/index.js

@@ -243,7 +243,7 @@ export type {
   ExecutionResult,
 } from './execution';
 
-export { subscribe } from './subscription';
+export { subscribe, getSubscriptionEventSource } from './subscription';
 
 // Validate GraphQL queries.
 export {

src/subscription/index.js

@@ -1 +1 @@
-export { subscribe } from './subscribe';
+export { subscribe, getSubscriptionEventSource } from './subscribe';