New WICG incubation: lock-screen

New API proposal - pursuant to
https://discourse.wicg.io/t/proposal-lock-screen-api/4372/3, I have created
a repo for https://github.com/WICG/lock-screen: an API for enabling web
apps on the lock screen of a device.   (This repo is currently empty, but
there are details in the Discourse thread.)