Upgrade Prettier from 1.17 to 2.0.2.

Summary:
This gets us on the latest Prettier 2.x:
https://prettier.io/blog/2020/03/21/2.0.0.html

Notably, this adds support for TypeScript 3.8,
which introduces new syntax, such as `import type`.

Reviewed By: zertosh

Differential Revision: D20636268

fbshipit-source-id: fca5833d003804333a05ba16325bbbe0e06d6c8a

desktop/app/src/utils/__tests__/jsonFileStorage.node.js

@@ -26,7 +26,7 @@ const storage = new JsonFileStorage(
 test('A valid settings file gets parsed correctly', () => {
   return storage
     .getItem('anykey')
-    .then((result) => expect(result).toEqual(validDeserializedData));
+    .then(result => expect(result).toEqual(validDeserializedData));
 });
 
 test('deserialize works as expected', () => {