OneSignal.login now returns auseable Promise

This function had a Promise for its return type but awaiting on it had
no effected before this commit.

Motivation for this change is to allow the developer to safely await
on login so follow up things like addEmail create the user with the
external_id correctly.

src/onesignal/OneSignal.ts

@@ -126,7 +126,7 @@ export default class OneSignal {
       );
     }
 
-    LoginManager.login(externalId, jwtToken);
+    await LoginManager.login(externalId, jwtToken);
   }
 
   static async logout(): Promise<void> {