feat: create stub for fileDownload

Create missing stub for lightning/fileDownload. Tackles #447

Author: FedeAbella

src/lightning-stubs/fileDownload/fileDownload.js

@@ -0,0 +1,8 @@
+/*
+ * Copyright (c) 2025, salesforce.com, inc.
+ * All rights reserved.
+ * SPDX-License-Identifier: MIT
+ * For full license text, see the LICENSE file in the repo root or https://opensource.org/licenses/MIT
+ */
+
+export const generateUrl = jest.fn().mockResolvedValue('');