Add new core extensions to the extensions helper

[richard67]

Pull Request for [SOC 21] Add a Scheduled Tasks Infrastructure to Joomla .

Summary of Changes

Core extensions have to be in the list of core extensions in the extensions helper so on an update of the CMS core, the post update script can update the manifest cache of these extensions in database.

[ditsuke]

Thanks! We'll need another entry for plg_task_checkfiles

[ditsuke]

Thanks!