Use comlink instead of jsonrpc layer for host<->extension communication

Created by: lguychard

We currently use our proprietary jsonrpc implementation for ext<->host communication.

The code is complex, and much boilerplate is needed when adding new features to the extension API (eg. progress).

We should replace the jsonrpc layer with comlink. This would allow us to expose the extension API inside the extension host worker with zero added boilerplate.

All credit goes to @felixfbecker