Add configuration options to `App` to choose reconciler

[carson-katri]

This PR enables support for App + Scene in the Fiber reconciler, and provides a way for apps to specify whether they want to use the FiberReconciler, and if they want to enable custom layout.

@main
struct MyApp: App {
  static let _configuration: _AppConfiguration = .init(
    reconciler: .fiber(useDynamicLayout: true)
  )

  var body: some Scene {
    WindowGroup {
      Text("Hello, world!")
    }
  }
}

[ezraberch]

I'd prefer this interface:

  static let _configuration: _AppConfiguration = .init(
    useReconciler: .fiber,
    shouldLayout: true
  )

[carson-katri]

I updated it to this:

static let _configuration: _AppConfiguration = .init(
  reconciler: .fiber(shouldLayout: true)
)

Reconciler options are:

.stack
.fiber()
.fiber(shouldLayout: true)

You can also pass the root environment in the configuration. Let me know what you think.

[carson-katri]

App should now properly handle @State.