Rename `Router` interface and remove dependency of Editor packages in channels

Today, Editor packages need to define their `Routes`, which end up being used by the Envelope and the Channels. This is not ideal, since the Editor code ends up in the channels bundles, making it bigger than it needs to be. This task is to remove the concept of a "Router", delegating to the channel the responsibility of knowing which file extensions it supports. Editor packages will then only be dependencies to Envelope apps.