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/core/package.json

@@ -290,6 +290,7 @@
     "@k8slens/application-for-electron-main": "^6.5.0-alpha.0",
     "@k8slens/button": "^1.0.0-alpha.5",
     "@k8slens/cluster-settings": "^6.5.0-alpha.1",
+    "@k8slens/cluster-sidebar": "^1.0.0-alpha.0",
     "@k8slens/error-boundary": "^1.0.0-alpha.5",
     "@k8slens/event-emitter": "^1.0.0-alpha.1",
     "@k8slens/kubectl-versions": "^1.0.0-alpha.0",