Enhancement: Skeleton App

The skeleton app has been part of the project for a long time, but with the conversion to vite it has since been ignored and no longer transpiled.
Due to the change, the app is now taken into account again, but must be explicitly enabled. For this please see the associated APP README.

Also new is a search example that is now included and uses GitHub to show how a custom search provider can be developed.

https://github.com/owncloud/web/pull/8441
