ggplot2
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Move {MASS} to Suggests
This PR aims to fix part of https://github.com/tidyverse/ggplot2/issues/5986#issuecomment-2217476203.
Briefly, MASS is required for 2D KDE and a particular ellipse type and not broadly all over ggplot2. This PR moves it to Suggests, adds checks whether it is installed in the relevant places and skips tests when it is not installed.