add Payment Request support?

[michielbdejong]

Probably an online shop would only ever accept LedgerLoops PrePay, but it would be cool to implement that. We could define the https://ledgerloops.com/prepay payment method and create a demo of how to use it.

It's a bit separate from the initial goal of this browser extension, though, which is to provide a demo of how LedgerLoops works, and give people a tool for it. The "ledger" link-rel already solves that. Adding Payment Requests would be a second way to achieve the same.