[w3c/payment-request] Add quantity prop to items (#861)

User may want to buy 2 or more items of a same product and the best way to show that to the user is by showing a `quantity` column.

This is a very common attribute in bills and receipts and could help users feel more confident when finishing a purchase, making it more clear.
Also, unit price would be required as well.

Please let me know what do you think about it?

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Received on Monday, 8 April 2019 19:27:23 UTC