Support FilesDirectories for file systems

[kevinslin]

Applications like Notion support "files as directories" - this is to say, you can create a file that has content but also has children. I'm currently working on an extension for VSCode with "files as directories" functionality but I'm not able to integrate this natively with the FileSystemProvider because Directory and File are distinct and non-overlapping types. This means that neither the file explorer or the breadcrumb are able to support my usecase.

A possible implementation:

• introduce a new FileType: FileDirectory
• support a new method FileSystemProvider.readFileDirectory
• when navigating via file explorer, default is to expand FileDirectory as Directory with right click option to open contents
• similar behavior for breadcrumb

[isidorn]

Forwarding to @jrieken because it is a FileSystemProvider feature request.