Add conveniences for working with the data_server backend

[joelostblom]

Currenlty, to show Altair plots created with the data_server enabled in other environments such as JupyterBook, I put something like this in my notebooks to fall back to an svg representation of the plot:

alt.renderers.enable('altair_saver', fmts=['vega-lite', 'svg'])

The downside with this is that it makes subsequent plots static, even if I switch from the data_server transformer to the default one and I need to manually change the renderer back with alt.renderers.enable('default') or alt.renderers.enable('altair_saver', fmts=['vega-lite', 'svg']) to show these plots.

I think it would be neat with a renderer format that inserts an html blurb only if the data_server is not activated (by checking alt.data_transformers.active and otherwise uses another format (could be called something like html_data_server). Even better would be if the default html format could somehow detect if there is not a live Python kernel running and if data_server was used, and if both those are true, remove the HTML spec instead of showing the empty chart)

[joelostblom]

Since Altair 5.2, the functionality of Altair Saver is now available in Altair via the vl-convert package. Most of the functionality has been available since 5.0, and the main addition in 5.2 was PDF export. See the docs on how to save charts for more details

We are going to archive this repo, so I'm closing all the open issues and PRs before doing so. Try out the new options for saving charts mentioned above and if you run into issues, please open an issue directly in the altair or vl-convert repo.