Sign of discount payment and list of payment discounts

A new property has been added to the V8Preview1 API for payment type IPaymentType.IsDiscount, which shows whether the payment type is discounted, i.e. carried out as a discount.

A list of discounts has also become available in the order IOrder.PaymentDiscounts, which is the result of applying order payments IOrder.Payments, that are discounted, i.e. carried out as a discount on the dishes ordered. Such payments are non-fiscal and are fiscalized on the FR side not as payments, but as discounts by reducing the cost of dishes.