Treat highway=busway as no-access by default

`highway=busway` is documented as having `access=no` and
`bus=designated` by default in the abscense of any overriding
access-tags.

In the main road query `access` is set to `no` when the road is a
busway, and access is either unset or contains `no` or `private`.

project.mml

@@ -723,6 +723,8 @@ Layer:
                 END AS int_surface,
                 CASE WHEN access IN ('destination') THEN 'destination'::text
                   WHEN access IN ('no', 'private') THEN 'no'::text
+                  -- highway=busway is access=no/bus=designated by default.
+                  WHEN highway IN ('busway') AND access IS NULL THEN 'no'::text
                 END AS access,
                 construction,
                 CASE