added classes for collections and bookmarks

lib/model/collection.dart

@@ -0,0 +1,12 @@
+import 'package:hive/hive.dart';
+
+class Collection extends HiveObject {
+  Collection({required this.name});
+
+  factory Collection.fromJson(Map<String, dynamic> json) =>
+      Collection(name: json['name'] as String);
+
+  String name;
+
+  Map<String, dynamic> toJson() => {'name': name};
+}