Add LottieView.animationDidLoad API

[calda]

This PR adds a new animationDidLoad API to the SwiftUI LottieView.

LottieAnimationView has an animationLoaded API, but its design makes it hard to use from SwiftUI. To support some specific implementation details of supporting both LottieAnimation and DotLottieFile in LottieAnimationView, the closure is called immediately when set if animation isn't nil:

/// A closure that is called when `self.animation` is loaded. When setting this closure,
/// it is called immediately if `self.animation` is non-nil.
///
/// When initializing a `LottieAnimationView`, the animation will either be loaded
/// synchronously (when loading a `LottieAnimation` from a .json file on disk)
/// or asynchronously (when loading a `DotLottieFile` from disk, or downloading
/// an animation from a URL). This closure is called in both cases once the
/// animation is loaded and applied, so can be a useful way to configure this
/// `LottieAnimationView` regardless of which initializer was used. For example:
///
/// ```
/// let animationView: LottieAnimationView
///
/// if loadDotLottieFile {
///   // Loads the .lottie file asynchronously
///   animationView = LottieAnimationView(dotLottieName: "animation")
/// } else {
///   // Loads the .json file synchronously
///   animationView = LottieAnimationView(name: "animation")
/// }
///
/// animationView.animationLoaded = { animationView, animation in
///   // If using a .lottie file, this is called once the file finishes loading.
///   // If using a .json file, this is called immediately (since the animation is loaded synchronously).
///   animationView.play()
/// }
/// ```
public var animationLoaded: ((_ animationView: LottieAnimationView, _ animation: LottieAnimation) -> Void)? {
  didSet {
    if let animation = animation {
      animationLoaded?(self, animation)
    }
  }
}

This doesn't work well when paired with .configure, because a call like this will invoke animationLoaded every time configure is called (which is arbitrarily often):

.configure { animationView in
  animationView.animationLoaded = {  _, _ in
    // handle the animation being loaded
    // (whoops, this is called way too often)
  }
}

In retrospect this was a bad API choice (imo) and we would have been better off with an API more similar to how LottieView works with LottieAnimationSource. Unfortunately we're stuck with this design on LottieAnimationView due to backwards compatibility.

To make this more obvious when using LottieView, lets just add a new animationDidLoad API with the expected semantics (only called after calling animationView.loadAnimation(animationSource)):

.animationDidLoad { _ in
  // handle the animation being loaded
}

[calda]

Removed this since it's out of date (AnimationPreviewViewController no longer exists)

[calda]

Xcode did this automatically 🤷🏻

[thedrick]

LGTM @calda