Add an interface to implement by DateTime and MutableDateTime

[olvlvl]

Just like PHP's DateTime and DateTimeImmutable implement the DateTimeInterface interface, ICanBoogie's DateTime and MutableDateTime should implement an interface.

Because we don't like the Interface suffix, and because we don't want to rename DateTime as ImmutableDateTime (although it would be more accurate), the interface should be defined as ICanBoogie\Contract\DateTime.