Implement Vega-Lite 4.0 transforms

[jakevdp]

Vega-Lite 4.0 adds a number of additional transforms, including density, loess, pivot, quantile, and regression. These can't be fully supported until Altair 4.0 comes out, but we could start implementing them before that.

[robna]

I was wondering if for these transforms the calculated values will now be accessible, as the current release of altair is 4.1.0? Especially I would like to extract the values of a density transform, but get the NotImplementedError.

My current way around is to calculate a KDE outside of altair (using SKLearn, as @jakevdp recommended here). But it would be great if I was able to do it from altair directly because of the easy groupby functionality it provides.

[jakevdp]

Some of these transforms have been implemented on master, but there hasn't been a release of this package for a while/