Support vega-lite :datasets key as well as :data

[athos]

Currently, the vega-lite viewer requires the data key in the selected value. However, the Vega-Lite specification doesn't actually require it, and the datasets field is sometimes useful rather than data (especially when you have an individual dataset for each layer).

This PR proposes to relax the current spec of the vega-lite viewer and change it so that it can accept either of the data or datasets key.

[djblue]

@athos thanks for the PR! I've never seen the or syntax in a keys spec, good to know 👍

[djblue]

@athos This should now be available in 0.46.0!

[athos]

Thanks for the quick release!