isLength function
check if the string's length falls in a range If no max is given then any length above min is ok.
Note: this function takes into account surrogate pairs.
Implementation
bool isLength(String str, int min, [int? max]) {
final surrogatePairs = _surrogatePairsRegExp.allMatches(str).toList();
int len = str.length - surrogatePairs.length;
return len >= min && (max == null || len <= max);
}