toUnicode method
Render to two-character text with unicode suit symbols
Implementation
String toUnicode() {
assert(isCard);
if (Card.WhiteJoker.index == index) return "Jw";
if (Card.BlackJoker.index == index) return "Jb";
if (Card.Joker.index == index) return "Jk";
return "${rank!.toChar()}${suit!.toUnicode()}";
}