[Tagging] Kerbs

[Selfish Seahorse]

I agree that `kerb:height` is more useful than `kerb`. However, `kerb`
seems to be a good starting point when mapping many kerbs and you
can't measure them all yet, as it gives a rough information whether
most wheelchair users can cross the street there or not.

The question is: does it make sense to introduce another `kerb` value
in order to differentiate between standard high kerbs and very high
kerbs at public transport stops? Or should common high kerbs be tagged
`kerb=raised` as well? The explanations on the wiki are contradictory
in that regard (e.g. 'older kerbs' vs 'raised above the norm').