Remove crossing=island option, add crossing:island field

[stragu]

As explained in the wiki:

The key crossing:island=* is used to specify whether a pedestrian crossing has a refuge island (also known as pedestrian island).
This key supersedes the former tag crossing=island.

This is because a crossing island is orthogonal to the rest of the crossing key classification (see also this section on the wiki).

It would be great to see crossing=island removed from the crossing field options, and have a crossing:island field added, similarly to the tactile paving field (i.e. switching between "unknown", "yes" and "no").

[stragu]

Thank you very much for the quick response! :)