letter function
a function that returns a random letter.
Implementation
String letter({
/// Default includes both upper and lower case. It's possible to specify one
/// or the other.
Casing? casing,
}) {
if (casing == Casing.lower) {
return _getRandomString(_lowerCaseChars);
} else if (casing == Casing.upper) {
return _getRandomString(_upperCaseChars);
}
return _getRandomString();
}