[Filters] Add filter to list items without dependencies

[lukmzig]

Extend modifier https://github.com/pimcore/generic-data-index-bundle/blob/1.x/src/Model/Search/Modifier/Filter/Dependency/RequiredByFilter.php
And provide a filter to list all items without dependencies

[github-actions]

Thanks a lot for reporting the issue. We did not consider the issue as "Pimcore:Priority", "Pimcore:ToDo" or "Pimcore:Backlog", so we're not going to work on that anytime soon. Please create a pull request to fix the issue if this is a bug report. We'll then review it as quickly as possible. If you're interested in contributing a feature, please contact us first here before creating a pull request. We'll then decide whether we'd accept it or not. Thanks for your understanding.