Include all files in the project's Resources subdirectory as BundleResource
items (except .DS_Store files, which are pretty omnipresent on macOS).
Also, contrary to the other default includes, add a condition so files are
only included if we have a resource prefix (typically "Resources"), otherwise
the entire hard drive might be included, and that's not really what we want.
Fixes https://github.com/xamarin/xamarin-macios/issues/13808.
Rolf Bjarne Kvinge