Move to Flutter 3.47, use the improvements that come with this (#64)

Reviewed-on: #64

lib/models/subspace.dart

@@ -1,10 +1,11 @@
 import "package:fast_immutable_collections/fast_immutable_collections.dart";
 import "package:freezed_annotation/freezed_annotation.dart";
 import "package:nexus/models/room.dart";
+
 part "subspace.freezed.dart";
 
 @freezed
-abstract class Subspace with _$Subspace {
-  const factory Subspace({required Room room, required IList<Room> children}) =
-      _Subspace;
-}
+class const Subspace({
+  required final Room room,
+  required final IList<Room> children,
+}) with _$Subspace;