add support for jumping to out-of-context events (#95)

Fixes #84.

Reviewed-on: #95

lib/models/room_chat.dart

@@ -0,0 +1,32 @@
+import "package:fast_immutable_collections/fast_immutable_collections.dart";
+import "package:freezed_annotation/freezed_annotation.dart";
+import "package:nexus/models/event.dart";
+
+part "room_chat.freezed.dart";
+part "room_chat.g.dart";
+
+@Freezed(toJson: false, fromJson: false)
+@JsonSerializable()
+class const RoomChat({
+  required final IList<Event> timeline,
+  required final bool hasMoreForward,
+  required final bool hasMoreBackward,
+  final HistoricalData? historicalData,
+}) with _$RoomChat {
+  Map<String, Object?> toJson() => _$RoomChatToJson(this);
+
+  factory RoomChat.fromJson(Map<String, Object?> json) =>
+      _$RoomChatFromJson(json);
+}
+
+@Freezed(toJson: false, fromJson: false)
+@JsonSerializable()
+class const HistoricalData({
+  required final String start,
+  required final String end,
+}) with _$HistoricalData {
+  Map<String, Object?> toJson() => _$HistoricalDataToJson(this);
+
+  factory HistoricalData.fromJson(Map<String, Object?> json) =>
+      _$HistoricalDataFromJson(json);
+}