Add bin to package.json (#24187)

Summary:
Add `bin` field to `package.json`. It will be required for initializing `react-native` with `npx`:
```sh
npx react-native init ProjectName
```

[General] [Added] - Added bin to package.json

cc thymikee cpojer
Pull Request resolved: https://github.com/facebook/react-native/pull/24187

Differential Revision: D14672211

Pulled By: hramos

fbshipit-source-id: 9e8633213bba4b3f02d20b54391a04d0a1a19376

cli.js

@@ -1,3 +1,4 @@
+#!/usr/bin/env node
 /**
  * Copyright (c) Facebook, Inc. and its affiliates.
  *

package.json

@@ -1,6 +1,7 @@
 {
   "name": "react-native",
   "version": "1000.0.0",
+  "bin": "./cli.js",
   "description": "A framework for building native apps using React",
   "license": "MIT",
   "repository": {