feat(firebase): add clearAllNotifications method (#2867)

* feat(firebase): add clearAllNotifications method

Added new method clearAllNotifications()

https://github.com/arnesson/cordova-plugin-firebase/blob/master/docs/API.md#clearAllNotifications

* remove trailing whitespace

src/@ionic-native/plugins/firebase/index.ts

@@ -338,4 +338,12 @@ export class Firebase extends IonicNativePlugin {
   ): Promise<any> {
     return;
   }
+
+  /**
+   * Clear all pending notifications from the drawer
+   * @return {Promise<any>}
+   */
+  clearAllNotifications(): Promise<any> {
+    return;
+  }
 }