Introduce broadcast communication

[jonatanklosko]

Adds Kino.Bridge.broadcast(topic, message) that goes through the group leader and is later broadcasted to interested processes on Livebook side. This way we can send events from JS server to Livebook as a single message and use the pubsub there, as opposed to sending the event to individual processes in Livebook.

Also adds Kino.Bridge.send(pid, message), so that all communication goes through the bridge API.

[jonatanklosko]

Added a Livebook-like group leader for tests, so that we can test parts relying on Kino.Bridge. Here specifically we want to assert that the events are broadcasted (while previously we would just subscribe the test process to the widget directly). Also added a number of functions shared across the Kino.JS.Live widgets, we can make it public once things settle (we will also need a clean way for people to use the new group leader for those tests).