Follow the XDG base directory specification for the .virtual_documents folder

[GergelyKalmar]

Elevator Pitch

The .virtual_documents folders should be created in the appropriate XDG base directory (either XDG_STATE_HOME or XDG_CACHE_HOME) instead of in the local folder.

Motivation

This is similar to the Python PYTHONPYCACHEPREFIX, which allows us to move all cache files out of our project tree structure, which can speed up test collection and other matters. It also makes cleaning up the cache easier and helps avoiding checking in these files accidentally into version control.

Design Ideas

Perhaps there is a workaround that could be implemented via virtual_documents_dir in a Python configuration, nonetheless, it would be great if this was supported out-of-the-box.