Get a file URL from a FileWrapper

FileWrapper+URL.swift

// Horrifying idea, but it seems to work on Apple platforms
// This is NOT stable across mounting/unmounting a file system

extension FileWrapper {
    var fileURL: URL? {
        let attr = self.fileAttributes
        guard let fileSystemNumber = attr["NSFileSystemNumber"] as? Int,
           let fileSystemFileNumber = attr["NSFileSystemFileNumber"] as? Int
        else { return nil }

        return URL(string: "file:///.file/id=\(fileSystemNumber).\(fileSystemFileNumber)/")
    }
}

Does not work on macOS, as it turns out.