testing: Add frontend tests.

[aero31aero]

Currently, we test the code examples in a node runtime. However, zulip-js also functions in the browser, and we should add bundling and testing in a browser based environment as well.

This task is open ended in terms of which bundler(s) to use and which testing framework to use. Ideally, we should be able to have a way to run all the currently existing tests in a stubbed out environment without the node utilities/packages available.

For reference: #47

[vaibhavrajsingh2001]

@aero31aero is this issue still open/requires work?

[aero31aero]

Yes, this is still available. There haven't been any updates on this front.