Add Twitch PubSub service

[hlxid]

Description

We should add a Twitch PubSub service to be able to react to e.g. channel points. Full list of supported events: https://dev.twitch.tv/docs/pubsub#topics. As a package we should use twitch-pubsub-client.
I think the currently existing nodecg-io-twitch service should be renamed with this change to nodecg-io-twitch-chat inorder to make it clear that it only handles the chat and not other parts of Twitch.

[derNiklaas]

Twitch PubSub is just using a websocket-client, so i dont think this is a major thing

[hlxid]

Agreed, but it would be still be nice to have this at some point so authentication can be done by the framework

[derNiklaas]

I just experimented with the package mentioned above. Looks easy to work with.
Small gist with example of the package
I'll try to finish it until thursday so @sebinside can use it on friday.