[w3c/browser-payment-api] Behavior for multiple modifiers for the same payment method is not defined (#309)

Currently the specification defines a `PaymentDetailsModifier` sequence which can be used to apply modifications to the total and display items as appropriate for the currently selected payment method:

https://w3c.github.io/browser-payment-api/#dom-paymentdetailsmodifier

However, the specification does not define the user agent behavior in the case where this sequence contains multiple `PaymentDetailsModifier` objects for the same payment method. This needs to be defined, otherwise user agents risk implementing differing behaviors that could result in incorrect amounts being charged.

I would suggest since this is a sequence stating the first entry to match a payment method will be used as its modifier.

Alternatively, we could say supplying multiple modifiers for a single payment instrument is not allowed and throw a `TypeError`.

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https://github.com/w3c/browser-payment-api/issues/309