Add bounds argument to `geom_density`

[MartinBernstorff]

Thanks a ton for the work on an excellent package! 🍾

When using geom_density, I get "drooping" edges because the geom assumes values outside the range of data are 0. More detail, as well as the PR to ggplot which fixed it with an argument, here: tidyverse/ggplot2#4013

[MartinBernstorff]

@HLasse You might find this interesting.