feat: Add experimental todo list for rooms

lib/pages/tasks/model/matrix_todo_list.dart

@@ -0,0 +1,59 @@
+import 'package:matrix/matrix.dart';
+
+extension MatrixTodoExtension on Room {
+  static const String stateKey = 'im.fluffychat.matrix_todos';
+  static const String contentKey = 'todos';
+
+  List<MatrixTodo>? get matrixTodos => getState(stateKey)
+      ?.content
+      .tryGetList(contentKey)
+      ?.map((json) => MatrixTodo.fromJson(json))
+      .toList();
+
+  Future<void> updateMatrixTodos(List<MatrixTodo> matrixTodos) =>
+      client.setRoomStateWithKey(
+        id,
+        stateKey,
+        '',
+        {contentKey: matrixTodos.map((todo) => todo.toJson()).toList()},
+      );
+}
+
+class MatrixTodo {
+  String title;
+  String? description;
+  DateTime? dueDate;
+  bool done;
+  List<MatrixTodo>? subTasks;
+
+  MatrixTodo({
+    required this.title,
+    this.description,
+    this.dueDate,
+    this.done = false,
+    this.subTasks,
+  });
+
+  factory MatrixTodo.fromJson(Map<String, Object?> json) => MatrixTodo(
+        title: json['title'] as String,
+        description: json['description'] as String?,
+        dueDate: json['due_date'] == null
+            ? null
+            : DateTime.fromMillisecondsSinceEpoch(json['due_date'] as int),
+        done: json['done'] as bool,
+        subTasks: json['sub_tasks'] == null
+            ? null
+            : (json['sub_tasks'] as List)
+                .map((json) => MatrixTodo.fromJson(json))
+                .toList(),
+      );
+
+  Map<String, dynamic> toJson() => {
+        'title': title,
+        if (description != null) 'description': description,
+        if (dueDate != null) 'due_date': dueDate?.millisecondsSinceEpoch,
+        'done': done,
+        if (subTasks != null)
+          'sub_tasks': subTasks?.map((t) => t.toJson()).toList(),
+      };
+}