Move virtuoso constant to proper location

src/lib/virtuoso/Virtuoso.constants.ts

@@ -0,0 +1,9 @@
+/*
+ * Copyright (C) Contributors to the Suwayomi project
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+export const GROUPED_VIRTUOSO_Z_INDEX = 2;