Use case - custom traceback renderer that provides links in output, which when clicked would:

• Go to relevant line/cell in notebook
• Provide user with option to install missing dependencies
• Extension to send message to renderer (with details of settings)
  • E.g. i had an extension that displays only the first n lines of the traceback.
  • Now extension can no longer pass this information to the renderer

I know there are other extension authors (Plotly) that are interested in looking at renderers with ability to communicate with extension host (not necessarily the Kernel).

I noticed yesterday that this was removed, and a few of my personal extensions no longer worked. Shipped a stripped version of the extsnion so we can see how I'm using some of the existing API (https://marketplace.visualstudio.com/items?itemName=donjayamanne.jupyterpowertools)

@connor4312 @rebornix /cc