fix(app): annotate initializeApp return as a promise (#8366)

Author: henribru

packages/app/lib/modular/index.d.ts

@@ -53,9 +53,9 @@ export function getApps(): FirebaseApp[];
  * Initializes a Firebase app with the provided options and name.
  * @param options - Options to configure the services used in the app.
  * @param name - The optional name of the app to initialize ('[DEFAULT]' if omitted).
- * @returns FirebaseApp - The initialized Firebase app.
+ * @returns Promise<FirebaseApp> - The initialized Firebase app.
  */
-export function initializeApp(options: FirebaseAppOptions, name?: string): FirebaseApp;
+export function initializeApp(options: FirebaseAppOptions, name?: string): Promise<FirebaseApp>;
 
 /**
  * Retrieves an instance of a Firebase app.