De-emphasize parking=street_side

For `amenity=parking` of the type `parking=street_side` (street-side
parking), the amenity icon is replaced with a smaller one that is shown
from z18 onwards.

With the introduction and formal acceptance of `parking=street_side`,
mappers now have a way of indicating that a mapped parking amenity is a
parking bay/area on the side of a street. This tag gives renderers a
chance to solve the problem of these parts of the street layout from
cluttering the map with large blue `P`'s that are better suited for
larger parking lots.

This commit introduces a smaller `P` icon that is pixel aligned to match
the crispness of the existing generic parking `P`.

Fixes gravitystorm#4262.

style/amenity-points.mss

@@ -1446,10 +1446,11 @@
   [feature = 'amenity_bicycle_parking'],
   [feature = 'amenity_motorcycle_parking'],
   [feature = 'amenity_parking_entrance'] {
-    [zoom >= 14][way_pixels > 750],
-    [zoom >= 17][feature = 'amenity_parking'],
+    [zoom >= 14][way_pixels > 750]["parking" != 'street_side'],
+    [zoom >= 17][feature = 'amenity_parking']["parking" != 'street_side'],
     [zoom >= 18] {
       [feature = 'amenity_parking'] { marker-file: url('symbols/amenity/parking.svg'); }
+      [feature = 'amenity_parking']["parking" = 'street_side'] { marker-file: url('symbols/amenity/parking_street_side.svg'); }
       [feature = 'amenity_bicycle_parking'] { marker-file: url('symbols/amenity/bicycle_parking.svg'); }
       [feature = 'amenity_motorcycle_parking'] { marker-file: url('symbols/amenity/motorcycle_parking.svg'); }
       [feature = 'amenity_parking_entrance']["parking"='underground'] { marker-file: url('symbols/amenity/parking_entrance_underground.svg'); }