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

RNTester/js/examples/SegmentedControlIOS/SegmentedControlIOSExample.js

@@ -118,13 +118,13 @@ class EventSegmentedControlExample extends React.Component<
     );
   }
 
-  _onChange = event => {
+  _onChange = (event) => {
     this.setState({
       selectedIndex: event.nativeEvent.selectedSegmentIndex,
     });
   };
 
-  _onValueChange = value => {
+  _onValueChange = (value) => {
     this.setState({
       value: value,
     });