fix: restore support for web workers

A previous change ([1], included in 3.0.5) broke support for web
workers, which threw "window is not defined" when importing the bundle
in a web worker.

Related: https://github.com/socketio/socket.io/issues/3809

Reference: https://webpack.js.org/configuration/output/#outputglobalobject

[1]: https://github.com/socketio/socket.io-client/commit/8c08c5d5c3bd6dd5477ca11a2625314da57724cc