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

Fixes #84.

Reviewed-on: #95

lib/models/event_context.dart

@@ -0,0 +1,22 @@
+import "package:fast_immutable_collections/fast_immutable_collections.dart";
+import "package:freezed_annotation/freezed_annotation.dart";
+import "package:nexus/models/event.dart";
+
+part "event_context.freezed.dart";
+part "event_context.g.dart";
+
+@Freezed(toJson: false, fromJson: false)
+@JsonSerializable()
+class const EventContext({
+  required final Event event,
+  required final IList<Event> before,
+  required final IList<Event> after,
+  required final String start,
+  required final String end,
+  final IList<Event> relatedEvents = const IList.empty(),
+}) with _$EventContext {
+  Map<String, Object?> toJson() => _$EventContextToJson(this);
+
+  factory EventContext.fromJson(Map<String, Object?> json) =>
+      _$EventContextFromJson(json);
+}