pusher-js: make available via global variable

Using Pusher via a CDN script tag registers the `Pusher` global. This change
allows for that global to be accessed/typed correctly.

See the docs on [UMD modules][1] for details.

[1]: https://www.typescriptlang.org/docs/handbook/modules.html#umd-modules

types/pusher-js/index.d.ts

@@ -228,4 +228,6 @@ declare namespace pusher {
 
 declare var pusher: pusher.PusherStatic;
 
+export as namespace pusher;
+
 export = pusher;