add "salt" as valid value for the surface tag

see https://github.com/openstreetmap/iD/issues/9305

data/fields/surface.json

@@ -16,7 +16,8 @@
             "grass": "Grass",
             "sand": "Sand",
             "wood": "Wood",
-            "artificial_turf": "Artificial Turf"
+            "artificial_turf": "Artificial Turf",
+            "salt": "Salt"
         }
     }
 }