added classes for collections and bookmarks

lib/model/bookmark.dart

@@ -0,0 +1,13 @@
+import 'package:hive/hive.dart';
+
+class Bookmark extends HiveObject {
+  Bookmark({required this.name, required this.link});
+
+  factory Bookmark.fromJson(Map<String, dynamic> json) =>
+      Bookmark(name: json['name'] as String, link: json['link'] as String);
+
+  String link;
+  String name;
+
+  Map<String, dynamic> toJson() => {'name': name, 'link': link};
+}