[Tagging] Feature Proposal - crossing=marked

[osm.tagging at thorsten.engler.id.au]

Forgot that part:

> On my walk yesterday, other than the implied crossing at every
> intersection (but see "don't map local law") I noted the following:

I do generally map crossings as long as they have lowered kerbs, like here:

https://cdn.discordapp.com/attachments/558999688670609448/577570543851536401/unknown.png

or even:

https://cdn.discordapp.com/attachments/558999688670609448/577571624585527296/unknown.png

https://www.openstreetmap.org/node/6465544169