NewGRF: Add town variables for uncapped house count, population and zone radii

Add feature name

src/newgrf_extension.h

@@ -87,6 +87,13 @@ enum Action2VariableRemapIds {
 	A2VRI_SIGNALS_SIGNAL_CONTEXT,
 	A2VRI_SIGNALS_SIGNAL_STYLE,
 	A2VRI_SIGNALS_SIGNAL_SIDE,
+	A2VRI_TOWNS_HOUSE_COUNT,
+	A2VRI_TOWNS_POPULATION,
+	A2VRI_ZONE_0,
+	A2VRI_ZONE_1,
+	A2VRI_ZONE_2,
+	A2VRI_ZONE_3,
+	A2VRI_ZONE_4,
 };
 
 enum GRFFeatureTestObservationFlag : uint8 {