charFrequency function
Determines the frequency of char in str.
filterNonChars removes non-chars from count.
Implementation
int charFrequency(String str, String char, {bool filterNonChars = true}) {
if (char.length != 1) return 0;
if (filterNonChars && !char.isChar) return 0;
final charCode = char.codeUnitAt(0);
return str.runes.where((rune) => rune == charCode).length;
}