Require Image for ConfettiType.image

[davdroman]

Xcode can generate static symbols from xcassets automatically, but they can't be used with this library because ConfettiType.image's input is currently a String. This PR changes the input type to Image instead:

// before (unsafe):
.confettiCannon(
  ...
  confettis: [.image("confetti")],
  ...
)
// after (safe):
.confettiCannon(
  ...
  confettis: [.image(Image(.confetti))],
  ...
)

I've deprecated the String and SFSymbol variants, but I'm happy to keep them if you so desire.