chore: Extract sidebar item injection token into separate package

- Add unit tests to cover custom resources sidebar items

Signed-off-by: Sebastian Malton <sebastian@malton.name>

packages/cluster-sidebar/index.ts

@@ -0,0 +1,8 @@
+/**
+ * Copyright (c) OpenLens Authors. All rights reserved.
+ * Licensed under MIT License. See LICENSE in root directory for more information.
+ */
+
+export * from "./src/tokens";
+export * from "./src/feature";
+export { default as sidebarItemsInjectable } from "./src/sidebar-items.injectable";