add isAfterMonth isAfterDay isBeforeMonth isBeforeDay

[utamori]

DateTIme class has isAfter isBefore
but There is no API to determine whether a given date is after or before the day after or before the day before.
also month

final DateTime specificDate = DateTime(2021, 01, 01);
final DateTime sameDate = DateTime(2021, 01, 01).add(const Duration(seconds: 5));
final DateTime otherDate = DateTime(2021, 01, 02);

print(specificDate.isAfterDay(sameDate)); // false
print(specificDate.isAfterDay(otherDate)); // true