refactor: expose reference sources independently of standalone setup

src/sources/reference.js

@@ -0,0 +1,10 @@
+import { createUsgsEarthquakeSource } from '../layers/earthquakes/source.js';
+import { createBundledCableSource } from '../layers/submarineCables/bundledSource.js';
+
+/** Construct the existing reference feeds independently of application setup. */
+export function createReferenceSources() {
+  return {
+    earthquakes: createUsgsEarthquakeSource(),
+    cables: createBundledCableSource(),
+  };
+}