pusher-js: make available via global variable
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

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