Discarding boxplot outliers

[teunbrand]

This PR aims to fix #4892.

Briefly it adds an outliers switch to geom_boxplot() that can turn outliers on or off, in a manner that does affect the scale ranges, in contrast to setting outliers.shape = NA.

Small demo:

devtools::load_all("~/packages/ggplot2")
#> ℹ Loading ggplot2

ggplot(mpg, aes(hwy, class)) +
  geom_boxplot(outliers = FALSE)

^{Created on 2023-08-03 with reprex v2.0.2}

[teunbrand]

It felt more appropriate to distinguish discarding vs hiding here than to keep the documentation over at the outlier.shape paragraph.

[thomasp85]

LGTM