[Woo POS] Mock`CartViewModel`

[iamgabrielma]

From review:

In SwiftUI preview, we could also pass a constant value like passing Just< PointOfSaleDashboardViewModel.OrderStage >(.building) to CartViewModel(orderStage:). We can potentially have two previews, one for each order stage case.

[dangermattic]

Thanks for reporting! 👍