Add self_checkout for shops

[lightonflux]

Wiki: https://wiki.openstreetmap.org/wiki/Key%3Aself_checkout

Taginfo: https://taginfo.openstreetmap.org/keys/self_checkout#overview

While this tag is still somewhat new and rarely used, i think Every Door is the right app to accelerate its adoption.

The tag itself does not lend itself to desktop mapping, as you need to visit a store to check. Nor does it fit StreetComplete (see here) as it cannot be tagged from the outside. But it would be a great addition to EveryDoor.

[Zverik]

I would be happy to see it in the editor: please create a pull request to https://github.com/openstreetmap/id-tagging-schema to add it to both iD and Every Door.