Refactor into monorepo for v6 (#1837)

> You can [learn more about this here](https://blog.invertase.io/react-native-firebase-2019-7e334ca9bcc6).

packages/app/lib/internal/FirebaseModule.js

@@ -0,0 +1,45 @@
+/*
+ * Copyright (c) 2016-present Invertase Limited & Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this library 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.
+ *
+ */
+
+import { getNativeModule } from './registry/nativeModule';
+import SharedEventEmitter from './SharedEventEmitter';
+
+export default class FirebaseModule {
+  constructor(app, config, customUrlOrRegion) {
+    this._app = app;
+    this._nativeModule = null;
+    this._customUrlOrRegion = customUrlOrRegion;
+    this._config = Object.assign({}, config);
+  }
+
+  get app() {
+    return this._app;
+  }
+
+  get emitter() {
+    return SharedEventEmitter
+  }
+
+  get native() {
+    if (this._nativeModule) return this._nativeModule;
+    this._nativeModule = getNativeModule(this);
+    return this._nativeModule;
+  }
+}
+
+// Instance of checks don't work once compiled
+FirebaseModule.__extended__ = {};