[w3c/browser-payment-api] PaymentResponse's details (#328)

I'm concerned about PaymentResponse's details attribute:

```IDL
  readonly attribute object          details;
```

Which is set as: 

> Set the details attribute value of response to a JSON-serializable object containing the payment method specific message that will be used by the merchant to process the transaction. The format of this response will be defined for each payment method.

Although the attribute is read-only, the object itself won't be. 

Maybe we want a `.getDetails()` method that always returns a new object - otherwise, this becomes racy if the `PaymentResponse` gets passed around. Developers could accidentally add new props to it, etc. which could cause weirdness in other parts of the application not expecting the value to change.

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