parking field: Add values on_kerb, half_on_kerb, shoulder (#672)

Following the successful proposal https://wiki.openstreetmap.org/wiki/Proposed_features/street_parking_revision#Summary:_What_is_proposed,_changed_and_deprecated? those keys are suggested for separately mapped parking areas. Since this field has `autoSuggestions` and `customValues` off they are listed all, even if not very common.

data/fields/parking.json

@@ -13,7 +13,10 @@
             "rooftop": "Rooftop",
             "sheds": "Sheds",
             "street_side": "Street-Side",
-            "layby": "Turnout / Lay-By"
+            "layby": "Turnout / Lay-By",
+            "on_kerb": "On Kerb",
+            "half_on_kerb": "Half On Kerb",
+            "shoulder": "Shoulder"
         }
     },
     "autoSuggestions": false,