add tombstone support

Fixes #147

lib/controllers/tombstone.dart

@@ -0,0 +1,22 @@
+import "package:flutter_riverpod/flutter_riverpod.dart";
+import "package:nexus/controllers/rooms.dart";
+import "package:nexus/models/content/content.dart";
+import "package:nexus/models/event.dart";
+
+class TombstoneController(final String roomId) extends Notifier<Event?> {
+  @override
+  Event? build() => ref.watch(
+    RoomsController.provider.select((rooms) {
+      final room = rooms[roomId];
+      return switch (room?.state[EventType.tombstone.type]?[""]) {
+        final rowId? => room?.events[rowId],
+        null => null,
+      };
+    }),
+  );
+
+  static final provider = NotifierProvider.family
+      .autoDispose<TombstoneController, Event?, String>(
+        TombstoneController.new,
+      );
+}