Add field self_checkout to shop preset

[lightonflux]

Description, Motivation & Context

This MR adds the established tag self_checkout to the shop preset.

Self checkouts are helpful for many people when shopping. E.g. when not speaking the local language or having social anxiety.

The tag is well-defined, and the adoption is accelerating. Adding it to the tagging schema would help mappers when using EveryDoor and similar mobile mapping apps (related issue: Zverik/every_door#774).

Self checkout counters are already established in the US and growing rapidly in many countries like Germany, and according to projections the number of them will grow rapidly in the coming years.

Links and data

Wiki: https://wiki.openstreetmap.org/wiki/Key:self_checkout

Tag info: https://taginfo.openstreetmap.org/keys/self_checkout