add invite support

Closes #82

lib/models/invited_room.dart

@@ -0,0 +1,16 @@
+import "package:fast_immutable_collections/fast_immutable_collections.dart";
+import "package:freezed_annotation/freezed_annotation.dart";
+import "package:nexus/models/matrix_event.dart";
+
+part "invited_room.freezed.dart";
+part "invited_room.g.dart";
+
+@Freezed(toJson: false, fromJson: false)
+@JsonSerializable(createToJson: false)
+class const InvitedRoom({
+  required final String roomId,
+  final IList<MatrixEvent> inviteState = const .empty(),
+}) with _$InvitedRoom {
+  factory InvitedRoom.fromJson(Map<String, Object?> json) =>
+      _$InvitedRoomFromJson(json);
+}