add: example for tanstack start

examples/tanstack-start/src/routes/api.$.ts

@@ -0,0 +1,13 @@
+import { createFileRoute } from "@tanstack/react-router";
+import config from "../../bknd.config";
+import { serve } from "bknd/adapter/tanstack-start";
+
+const handler = serve(config);
+
+export const Route = createFileRoute("/api/$")({
+  server: {
+    handlers: {
+      ANY: async ({ request }) => await handler(request),
+    },
+  },
+});