[w3c/browser-payment-api] maybe add `DisplayItem` dictionary (#457)

As I was implementing using the API on a site, I found I needed to gather together "display items" (`PaymentShippingOption` and `PaymentItem`) in order to calculate tax (`PaymentItem`). Critical for displaying these to the user is the `label` and `amount`. 

As such,  think we should make an Editorial change to introduce:

```JS
dictionary DisplayItem : Localizable {
    required DOMString             label;
    required PaymentCurrencyAmount amount;
};

// and so... 
PaymentShippingOption : DisplayItem {
   // other members
}
PaymentItem : DisplayItem {
  // other members
}
```

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