[w3c/payment-request] Way of detecting if can make payment (#801)

Follow up of #800. 

We need to add a means to answer "If `canMakePayment` is true, is there a valid credential that could be used to make a payment?"

-- 
You are receiving this because you are subscribed to this thread.
Reply to this email directly or view it on GitHub:
https://github.com/w3c/payment-request/issues/801