New WICG incubation: Payment link type in HTML

Pursuant to the proposal thread at
https://github.com/WICG/proposals/issues/150, we've adopted
https://github.com/WICG/paymentlink into WICG.  This incubation is for a
'passive' method to allow browsers to detect push payment flows, to allow
users of integrated wallets the option of a better experience.

-Chris