Add @ViewBuilder to NavigationStack root view

[joshrl]

Adding view builder annotation to NavigiationStack. init. This matches the functionality of the built in initializer's root view. See #3483

Note that I'm not sure if there is a way to add a test for this exactly, but what I did to confirm was to modify NavigationDemoView in SwiftUICaseStudies locally (not checked in) like so:

  var body: some View {
    NavigationStack(path: $store.scope(state: \.path, action: \.path)) {
      Text("Fred")
      Form {
        Section { Text(template: readMe) }
      ...

[stephencelis]

Thanks!