Re-export singleton object for internal-override.

Internal packages like firebase-functions-test relies on export of functions config singleton object to mock/override its behavior. We should keep this setup (with an @internal mark so it isn't exposed in type or reference doc) to allow these packages to continue to work.

Author: taeold

src/v1/config.ts

@@ -25,7 +25,8 @@ import * as path from "path";
 
 export { firebaseConfig } from "../common/config";
 
-let singleton: Record<string, any>;
+/** @internal */
+export let singleton: Record<string, any>;
 
 /** @internal */
 export function resetCache(): void {