Dart SDK support

[jimmyff]

Hey, awesome work on this, just had a play and it's great!

Just wondering about the decision to make this a Flutter package rather than a Dart package? Dart packages can be used in Flutter apps but Flutter packages cannot be used in Dart apps. As this has no UI a Dart package might be a better fit? If it's due to the asset loading you could either leave that to the user or you could create a sweph-flutter package which uses the core sweph dart package but adds on flutter asset loading? Is it something you could consider? It would be great to be able to use this in cloud-functions etc.

Thanks!