Globally accessible client

[tombruijn]

Like issue appsignal/appsignal-nodejs#503 for Node.js, make the client available globally. For JavaScript that would be window probably.

Let's discuss the best way to implement this.

[tombruijn]

Is there a problem with putting everything on the global namespace? Because people can run multiple JavaScript apps in the frontend, and they may want to report them as separate apps?
If we go with the global way, this won't be supported, which isn't bad, but it's something to keep in mind.