changeToVertical method
Implementation
Future<String> changeToVertical () async {
// SAVE HTML STRING TO LOCAL
var directory = await getApplicationCacheDirectory();
var verticalString = OrientationUtils.verticalStringSummon(bookContents!);
var verticalFile = File('${directory.path}/temporary.html');
// WRITE TO THE FILE
var resultant = await verticalFile.writeAsString(verticalString);
var result = resultant.path;
currentHtmlString = result;
return result;
}