Add aliases to make program types more beginner friendly

[JoelQ]

The return types of picture, animation, and game are pretty intense for newcomers, particularly those not super comfortable with advanced type signatures. Additionally, they are hard-coded with internal types which causes confusion. I think hiding that behind a type alias would make the experience much more approachable. Something like:

type alias PictureProgram = 
  Platform.Program () Screen ( Basics.Int, Basics.Int )

type alias AnimationProgram =
  Platform.Program () Animation Msg

type alias GameProgram memory =
  Platform.Program () (Game memory) Msg

This would allow folks to write programs like:

main : PictureProgram
main =
  picture [ circle blue 40 ]

or

main : AnimationProgram
main =
  animation view

or

main : GameProgram Memory
main =
  game view update initialMemory