notebook kernel messaging and renderer messaging fixes

[jonah-iden]

What it does

This implements the proposed Kernel messaging API (vscode.proposed.notebookMessaging.d.ts) and the notebookPreload contribution point. Also fixes notebook renderer messaging. This is to support interactive output widgets like for the ipywidgets library.

Tested with ipywidgets 7.8.1 and 8.1.2

How to test

1. pip install ipywidgets in your used kernel
2. Open a notebook
3. add following cell

import ipywidgets as widgets
widgets.IntSlider(min=1, max=12)

4. Execute the cell and see the interactive slider is rendered

Follow-ups

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[msujew]

This looks pretty good. I've confirmed that this works even for pretty complicated widgets such as:

a = widgets.FloatText()
b = widgets.FloatSlider()
display(a,b)

mylink = widgets.jslink((a, 'value'), (b, 'value'))

Before approving, I wanted to ask about this error. Have you seen that one when trying to execute a ipywidgets cell? If so, do you think it makes sense to fix it or leave it for a separate PR (I'd be fine with it, as it doesn't seem to impact behavior):

[jonah-iden]

Thanks for the review. No, I did not see that fallbackToNextRenderer when testing ipywidgets.
since its coming from the ipywidgets renderer it could also be that there may be a python library missing or something like that
I think we can leave that for a separate PR

[msujew]

Thanks, looks good to me 👍