fix(types): add missing typedefs for bulkSave() to 5.x

Fix #12019

Author: vkarpov15

index.d.ts

@@ -803,6 +803,13 @@ declare module 'mongoose' {
  bulkWrite(writes: Array<any>, options?: mongodb.CollectionBulkWriteOptions): Promise<mongodb.BulkWriteOpResultObject>;
  bulkWrite(writes: Array<any>, options?: mongodb.CollectionBulkWriteOptions, cb?: Callback<mongodb.BulkWriteOpResultObject>): void;
 
+ /**
+ * Sends multiple `save()` calls in a single `bulkWrite()`. This is faster than
+ * sending multiple `save()` calls because with `bulkSave()` there is only one
+ * network round trip to the MongoDB server.
+ */
+ bulkSave(documents: Array<Document>): Promise<mongodb.BulkWriteResult>;
+
  /** Collection the model uses. */
  collection: Collection;