Support rendering of Vega and Vega Lite on Github markdown and Jupyter notebooks

[joelostblom]

codi.md renders vega-lite specs in their markdown documents. It would be great if this was supported in GitHub markdown as well:

It would also be very beneficial if Vega Lite and Altair charts were rendered in Jupyter notebooks instead of just showing a blank output which makes it hard to review pull requests that contain Vega charts.

[joelostblom]

I wrote a comment with more details regarding Vega-Lite specs generated from Altair in Jupyter Notebooks, including some example notebooks in a public repo; you can view it here https://github.com/orgs/community/discussions/37376#discussioncomment-7159686