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: socketio/socket.io#3809

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

[1]: 8c08c5d

Author: darrachequesne

support/webpack.config.js

@@ -11,6 +11,7 @@ module.exports = {
     filename: "socket.io.js",
     library: "io",
     libraryTarget: "umd",
+    globalObject: "this",
   },
   mode: "development",
   devtool: "source-map",