Undeprecate preview traits?

[stephencelis]

I believe I stumbled onto a fix for preview code bleeding between previews, so this works in a single file:

struct MyView: View {
  @Dependency(\.date.now) var now
  var body: some View {
    Text("\(now)")
  }
}

#Preview(
  "Controlled",
  traits: .dependency(\.date.now, Date(timeIntervalSince1970: 0)
) {
  MyView()
}

#Preview("Uncontrolled") {
  MyView()
}

The one caveat to this is that if you access a dependency directly in the #Preview block you must use @Previewable @Dependency, but that's the same for @Environment.

[stephencelis]

Introducing a preview modifier allows us to delay when dependencies are prepared to the computation of the preview's body.