isHexadecimal function
Checks if the string is a hexadecimal number.
Accepts an optional 0x or 0h prefix followed by one or more
hexadecimal digits.
Example:
isHexadecimal('deadBEEF'); // true
isHexadecimal('0xff'); // true
isHexadecimal('xyz'); // false
Implementation
bool isHexadecimal(String str) => _isHexadecimal(str);