Real-time collaboration

A collaboration feature using Websockets, preferably in a plugin that’s backed by the Core team, would be a nice addition for teams working on larger Kirby sites

Related to https://kirby.nolt.io/536

It seems fine to me to use a third party provider like Pusher to provide the webserver part if it stays as a plugin. Maybe they could be a way to provide barebones communication framework in Core and let the actual client side communication happen via a plugin.