feat: read the transit feed catalog through the transit source

Add getFeeds() to the transit source. It reads the public catalog route
the server already serves, so code outside the layer can find feeds
near a location without loading the layer.

src/layers/transit/source.js

@@ -5,6 +5,21 @@ export function createTransitSource({
   fetchImpl = (...args) => fetch(...args),
 } = {}) {
   return {
+    /** Read the public catalog of enabled feeds. */
+    async getFeeds({ signal } = {}) {
+      signal?.throwIfAborted();
+      const response = await fetchImpl('/api/transit/feeds', {
+        signal,
+        headers: { Accept: 'application/json' },
+      });
+      if (!response.ok)
+        throw new Error('Transit feeds HTTP ' + response.status);
+      const body = await response.json();
+      signal?.throwIfAborted();
+      if (!Array.isArray(body?.feeds))
+        throw new Error('Malformed transit feed catalog');
+      return body.feeds;
+    },
     getHistory(feedId, vehicleId, { signal } = {}) {
       signal?.throwIfAborted();
       return fetchTransitHistory(feedId, vehicleId, signal, fetchImpl);