fix: add extension in the package.json main entry (#608)

The `main` entry for the `package.json` file is for defining the entry
point. The entry is currently missing the required `.js` extension,
this PR brings it back.

Backported from 17b8c2f

Author: AStoker

Diff for: package.json

@@ -2,7 +2,7 @@
   "name": "engine.io",
   "version": "3.5.0",
   "description": "The realtime engine behind Socket.IO. Provides the foundation of a bidirectional connection between client and server",
-  "main": "lib/engine.io",
+  "main": "lib/engine.io.js",
   "author": "Guillermo Rauch <guillermo@learnboost.com>",
   "homepage": "https://github.com/socketio/engine.io",
   "contributors": [