Add 2D KDE plots

[jordandeklerk]

I don't think we have 2D KDE contour plots right now. Maybe this could be added to plot_dist() via the kind arg? We can also address this arviz-devs/arviz#2473.

[aloctavodia]

Yes we need to add it. plot_dist plots 1D marginals. We can extend plot_pair and plot_pair_focus. I think we don't even have 2d kde in arviz-stats, we should add it there first.