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