last method
Gets the last len characters of this string, handling Unicode correctly.
Returns the full string if len is greater than the string length.
Implementation
String last(int len) {
if (isEmpty || len <= 0) return '';
if (len >= runes.length) return this;
final List<int> runeList = runes.toList().sublist(runes.length - len);
return String.fromCharCodes(runeList);
}