Add basic side menu for layer selection

This _should_ be smart and not load expensive layers until they're rendered.

layers/index.js

@@ -0,0 +1,35 @@
+import TileLayer from 'ol/layer/Tile';
+import OSM from 'ol/source/OSM';
+
+import amtrakLayer from './amtrak/layer.js';
+import arenasLayer from './nhl-arenas/layer.js';
+
+const layerCategories = [
+    {
+        name: "Base maps",
+        layers: [
+            {
+                name: "OpenStreetMap Carto",
+                enabled: true,
+                layer: new TileLayer({
+                    source: new OSM(),
+                }),
+            },
+        ],
+    },
+    {
+        name: "Overlays",
+        layers: [
+            {
+                name: "Amtrak Routes",
+                layer: amtrakLayer,
+            },
+            {
+                name: "NHL Arenas",
+                layer: arenasLayer,
+            },
+        ]
+    }
+];
+
+export default layerCategories;