carto: extract polygonArea helper

app/javascript/old_design/carto/polygon_area.js

@@ -0,0 +1,19 @@
+import area from '@turf/area';
+
+export default function polygonArea(latLngs) {
+  return area({
+    type: 'FeatureCollection',
+    features: latLngs.map(featurePolygonLatLngs)
+  });
+}
+
+function featurePolygonLatLngs(latLngs) {
+  return {
+    type: 'Feature',
+    properties: {},
+    geometry: {
+      type: 'Polygon',
+      coordinates: [latLngs.map(({ lng, lat }) => [lng, lat])]
+    }
+  };
+}