[w3c/payment-request] Merchant validation (#646)

We need a means to do merchant validation, as per:
https://developer.apple.com/documentation/applepayjs/providing_merchant_validation

However, this is "one and done" event, so it's a good candidate to be a promise. So, sketch:

```JS
const pr = new PaymentRequest();
// Still don't know what a website does with validationURL? 
const validationURL = await pr.validateMerchant(); 
await pr.show();  
```



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