Allow returning a directory with both read and write permissions in a single prompt for the File System Access API. Currently showDirectoryPicker() always returns a read-only directory (after showing a read access prompt), requiring a second prompt to get write access. This double-prompt is a poor user experience and contributes to confusion and permission fatigue among users. Adds an optional "mode" option to DirectoryPickerOptions which can be specified as either "read" or "readwrite".