File Importer Support

[JamesSedlacek]

Support File Importer (presents a system interface for allowing the user to import multiple files)

https://developer.apple.com/documentation/swiftui/view/fileimporter(ispresented:allowedcontenttypes:allowsmultipleselection:oncompletion:)

The presenter function would be something like this:

presenter.presentFileImporter { result in 
    // onCompletion 
}