Add file missing in previous commit

src/backends/browser.js

+/* Copyright 2021 Dominik George <nik@naturalnet.de>
+ *
+ * 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.
+ */
+
+import { BaseBackend, EventTypes } from './base';
+import * as sdk from "matrix-js-sdk";
+
+class BrowserBackend extends BaseBackend {
+  constructor(userId, homeServerUrl) {
+    super(userId);
+
+    this.api = sdk.createClient(homeServerUrl);
+  }
+}
+
+export default WidgetBackend;