Add a companion package for the host page

[jtpio]

The companion package will be developed from the same repo.

It will:

• provide an API to send messages to the embedded IFrame
• allow taking an id of an IFrame as parameter, to know to which IFrame to send messages to
• default to using the first IFrame if not configured otherwise
• be packaged as an independant JavaScript package, so it's easier to reuse in other projects

[jtpio]

Posting more thoughts here for the companion package:

• we can structure the repo as a monorepo with:
  • packages/jupyterlab-iframe-commands-host: the companion package to be used on the host
  • packages/jupyterlab-iframe-commands-extension: the JupyterLab extension that will be enabled in the IFrame
  • at the top-level, we can keep the Python package which is only used for packaging and distributing the JupyterLab extension via PyPI
  • use lerna for consistency with other repos, and to be able to build everything easily from the root folder
• the demo in this repo would make use of both the companion package and the JupyterLab extension
• consumers of the companion package will be able to use with something like this in their code (as an example):

import { CommandBridge } from 'jupyterlab-iframe-commands-host';

const bridge = new CommandBridge({ iframeId: 'jupyterlab' });
await bridge.execute('test-command');