RFE: Add support for File System Access API

[kisielk]

The File System Access API allows a web application to read and write files on a user's local filesystem.

By using this API the FaustIDE could read / write files directly from / to a user's machine, allowing it to act more like a native locally running application. I think this would be a major workflow improvement for a lot of people that currently need to copy / paste code between a local editor and the web IDE.