message: Enhcance tests for clientDocumentMixin
diff: @@ -13,3 +13,6 @@ expectType<ActiveEffect[]>(doc.transferredEffects); // class itself
  expectType<string>(Item.documentName); // Document
  expectType<typeof data.ItemData>(Item.schema); // Base-Document
  expectType<Promise<Item>>(Item.createDialog()); // ClientDocumentMixin
  +
  +// Properties
  +expectType<ItemSheet | null>(doc.sheet);
  