Add mock class in fs api

Author: CodeDoctorDE

packages/lw_file_system_api/lib/lw_file_system_api.dart

@@ -6,5 +6,6 @@ library;
 export 'src/entity.dart';
 export 'src/name.dart';
 export 'src/location.dart';
+export 'src/mock.dart';
 export 'src/storage.dart';
 export 'archive.dart';

packages/lw_file_system_api/lib/src/mock.dart

@@ -0,0 +1,9 @@
+import 'storage.dart';
+
+class MockPasswordStorage implements PasswordStorage {
+  @override
+  Future<String?> read(ExternalStorage storage) async => null;
+
+  @override
+  void write(ExternalStorage storage, String password) {}
+}