fix(bundle): restore support for JS modules

This change is needed so the bundle can be used with:

```
<script type="module" src="xxxx/socket.io.js"></script>
```

Related:

- socketio/socket.io#3828
- 13b32b3
- 8c08c5d

Author: darrachequesne

support/webpack.config.js

@@ -11,7 +11,7 @@ module.exports = {
     filename: "socket.io.js",
     library: "io",
     libraryTarget: "umd",
-    globalObject: "this",
+    globalObject: "self",
   },
   mode: "development",
   devtool: "source-map",