Support FilesDirectories for file systems

<!-- ⚠️⚠️ Do Not Delete This! feature_request_template ⚠️⚠️ --> <!-- Please read our Rules of Conduct: https://opensource.microsoft.com/codeofconduct/ --> <!-- Please search existing issues to avoid creating duplicates. --> <!-- Describe the feature you'd like. --> Applications like [Notion](https://notion.so) 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