Fix missing "os" polyfill

Summary:
Webpack 5 no longer polyfills "os".
Force bundles to provide an empty module for "os" for browsers.

Reviewed By: antonk52

Differential Revision: D35114196

fbshipit-source-id: 82dd2dc9e15dafc595f9242cdb2f3cb1ddf07268

js/js-flipper/package.json

@@ -3,6 +3,9 @@
   "title": "JS Flipper Bindings for Web-Socket based clients",
   "version": "0.140.0",
   "main": "lib/index.js",
+  "browser": {
+    "os": false
+  },
   "types": "lib/index.d.ts",
   "description": "Flipper bindings for Node.js and web",
   "scripts": {