Make JS versions more inspectable

[jakevdp]

It would be nice to provide an easy means for the user to see what version of vega/vega-lite is installed with this package.

Currently (unless I'm mistaken) the only way is to look at package.json, find the vega-embed version, then go to the correct vega-embed release, open its package.json, and look for the vega-lite version there.

I'd love something similar to

>>> import vega
>>> vega.js_versions
{'vega': '4.0.0',
 'vega-lite': '2.6.0',
 'vega-embed': '3.16.0'}

The trick is to do that without too much manual bookkeeping...

[domoritz]

If we had a way to communicate back from the JavaScript to the kernel, this would be easier.

[jakevdp]

Is there a way to generate files as part of the npm build step?

[jakevdp]

Or maybe just something like console.logging the versions on load?

[domoritz]

It would be nice to if we don't have to add another build step to compile the versions into the python code. Logging the versions on load is trivial.