add method

Implementation

@override
Gregorian add({int years = 0, int months = 0, int days = 0, int hours = 0, int minutes = 0, int seconds = 0, int milliseconds = 0}) => Gregorian(
      year + years,
      month + months,
      day + days,
      hour + hours,
      minute + minutes,
      second + seconds,
      millisecond + milliseconds,
    );