feat: deprecate SwiftUIScreen (#336)

To begin phasing out SwiftUIScreen & WorkflowSwiftUIExperimental in
favor of ObservableScreen & WorkflowSwiftUI.

Author: watt

WorkflowSwiftUIExperimental/Sources/SwiftUIScreen.swift

@@ -20,6 +20,7 @@ import SwiftUI
 import Workflow
 import WorkflowUI
 
+@available(*, deprecated, message: "Use the ObservableScreen protocol from WorkflowSwiftUI")
 public protocol SwiftUIScreen: Screen {
     associatedtype Content: View