[Woo POS] Improve testability: Consider change unique ID for `CartItem`

[iamgabrielma]

Currently, we initialize CartItem within addItemToCart(:), and provide it with an unique ID. This acts as unique identifier to differentiate between multiple items in the cart that could share the same product ID

    func addItemToCart(_ item: POSItem) {
        let cartItem = CartItem(id: UUID(), item: item, quantity: 1)
        itemsInCart.append(cartItem)
    }

We should consider if, for testability, we should create the CartItem and it's ID not as a side effect of "adding items to cart", but as a separate method. On the other end, this UUID is exclusively of internal usage for the CartView, so perhaps is not needed to expose it.

[dangermattic]

Thanks for reporting! 👍