Add exit handler for Node.js environments, fix #51 #55

[hanspagel]

This PR introduces:

1. A check if the provider is running in a browser or Node.js context.
2. Registers an exit handler for Node.js environments.

The check isn’t very strict, but I don’t see a reason why someone would benefit from manipulating the check. From what I know it’s pretty complex to make that check harder to manipulate.

A very similar change in lib0 is necessary to make y-websocket work in Node.js (again?). I’ll send a PR there too. Let me know if a change to the PR would make it easier to merge it. Happy to update it.