Improve unused check for previews

[bsudekum]

If you have something like:

#Preview {
  let viewModel = ViewModel()
  MyView(viewModel: viewModel)
}

and MyView is actually used elsewhere, viewModel is called out as unused. I'd rather not enable retain-swift-ui-previews because then if MyView is actually unused, it wouldn't get called out as unused.

The ideal situation is viewModel is not called out as unused, bc it is technically used in the preview.