Browser SDK: IIFE compatibility

[sdirosa]

We recently received a troubleshooting request for loading Browser Tracker in a non-modularized vanilla JS application.

This is currently not possible, as Browser Tracker was designed to function in modularized ES6 at least.

That said, we can improve things by exposing the Browser SDK API to the Objectiv Global object and we probably need to add a couple of helpers to make it easier to tag elements from <script> or script modules.