Add wasHole property to DateTime

[icambron]

When you create a DateTime on a hole (i.e. a local time that does not exist), Luxon detects that and bumps it forward. The issue is that we never tell the user we did this. This PR adds a wasHole property to the DateTime to signal that's what happened.

[diesieben07]

I am not sure I am a fan of the API here, because this really is not a property of the DateTime (which manifests in the fact that you have to make sure to remove it on setZone, etc.).
But I cannot think of a better API either, because basically any factory would have to return this. I will think about it some more though before I merge this.

[icambron]

I agree it's more like metadata for the factory process than a property of the datetime but there isn't a great way to signal that unless we return more complicated objects from those factory functions. Which doesn't seem reasonable.