feat: Check markdown checkboxes in messages

lib/utils/event_checkbox_extension.dart

@@ -0,0 +1,27 @@
+import 'package:matrix/matrix.dart';
+
+extension EventCheckboxRoomExtension on Room {
+  static const String relationshipType = 'im.fluffychat.checkboxes';
+  Future<String?> checkCheckbox(
+    String eventId,
+    int checkboxId, {
+    String? txid,
+  }) =>
+      sendEvent(
+        {
+          'm.relates_to': {
+            'rel_type': relationshipType,
+            'event_id': eventId,
+            'checkbox_id': checkboxId,
+          },
+        },
+        type: EventTypes.Reaction,
+        txid: txid,
+      );
+}
+
+extension EventCheckboxExtension on Event {
+  int? get checkedCheckboxId => content
+      .tryGetMap<String, Object?>('m.relates_to')
+      ?.tryGet<int>('checkbox_id');
+}