add "kerb" tag as an optional field to crossing vertices

when used without `barrier=kerb`, but on a crossing node.

see also https://wiki.openstreetmap.org/w/index.php?title=Key:kerb&oldid=2501049#By_itself

interim/source_strings.yaml

@@ -2711,6 +2711,7 @@ en:
       kerb:
         # kerb=*
         label: Curb
+        terms: '[translate with synonyms or related terms for ''Curb'', separated by commas]'
       kerb/height:
         # kerb:height=*
         label: Height