add an empty typings file to firebase (#5671)

* add an empty typings file to firebase

* formatting

Author: Feiyang1

packages/firebase/empty.d.ts

@@ -0,0 +1,18 @@
+/**
+ * @license
+ * Copyright 2021 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+export {};

packages/firebase/package.json

@@ -274,5 +274,6 @@
     "messaging",
     "messaging/sw",
     "database"
-  ]
+  ],
+  "typings": "empty.d.ts"
 }