slice method
Implementation
String slice(int start, [int? end]) {
if (isEmptyOrNull) return '';
var s = start, e = end ?? size;
if (s < 0) {
s = max(0, size + s);
}
if (e < 0) {
e = max(0, size + e);
}
return (s < e) ? this!.substring(s, min(e, size)) : '';
}