Upgrade to TypeScript 4.1

Summary:
allow-large-files

Upgrade to TypeScript 4.1

Reviewed By: nikoant

Differential Revision: D25122073

fbshipit-source-id: 7d2bc6f97595c825c9527b0c6ecab07698d5f981

desktop/plugins/shared_preferences/src/__test__/shared_preferences.spec.tsx

@@ -11,7 +11,7 @@ import {TestUtils} from 'flipper-plugin';
 import * as plugin from '..';
 
 async function sleep(ms: number) {
-  return new Promise((resolve) => setTimeout(() => resolve(), ms));
+  return new Promise<void>((resolve) => setTimeout(() => resolve(), ms));
 }
 
 // this testing is inspired by Flipper sample app