Add support for 2d kernel density contours.

[jbengler]

Discussed in #44

^{Originally posted by ar-puuk January 13, 2025}
Hi,

I was checking a python implementation of tidyplots (here), and saw a function to add a 2d kernel density estimate contours over scatter plot (equivalent to ggplot2::geom_density_2d() or ggplot2::stat_density_2d()). Would it be possible to add a similar function to tidyplots?

Appreciated.