isCryptoCurrency function
Basically it doesn't really check if the currencyId is a crypto currency. It just checks if it's not fiat. I decided not to collect all possible crypto currecies as there's an endless amount of them
Implementation
bool isCryptoCurrency(String currencyId) {
if (currencyId.length < 3 || currencyId.length > 4) {
return false;
}
return !isFiatCurrency(currencyId);
}