Enable IFrame -> host messaging

[jtpio]

For the first version, the JupyterLab extension defines an API for accessing commands, and exposes it to the host page with Comlink's expose helper.

At some point in the future, it may be interesting to enable communication the other way around: the host exposes an API, and the extension is able to call it at any time during the application lifecycle.

Posting here some notes on how this could be achieved with the comlink API.

Host

const endpoint = Comlink.windowEndpoint(childWindow);
// sample host API
const hostApi = {
  async getCmds() {
    return ['command1', 'command2', 'command3'];
  }
};

Comlink.expose(hostApi, endpoint);

IFrame

const endpoint = windowEndpoint(self.parent);
const host = wrap(endpoint) as any; // TODO: fix typings?
const cmds = await host.getCmds();
console.log('Commands from host:', cmds);