Use showDirectoryPicker when supported #1251
Description
The current implementation shows an "upload" warning dialog makes it seem like I am sending all my mp3 files to the hosted site :
when in fact I am just giving the web app access to those local files.
showDirectoryPicker is supported in a significant portion of browsers (80% of desktop users). It provides a nicer experience by saying "Let site view files?" :
showDirectoryPicker() just returns a Promise<FileSystemDirectoryHandle>