ggdist: Visualizations of distributions and uncertainty

Initial split from tidybayes: ggdist now contains all stats/geoms from tidybayes (except deprecated ones), all support functions for stats/geoms (such as point_interval()), vignette("slabinterval"), and vignette("freq-uncertainty-vis"). Tidybayes will retain all other functions, and will re-export all ggdist functions for now. All stats and geoms now support automatic orientation determination. Thus, all h-suffix geoms are now deprecated. Those geoms have been left in tidybayes and give a deprecation warning when used; they cannot be used from ggdist directly. geom_interval(), geom_pointinterval(), and geom_lineribbon() no longer automatically set the ymin and ymax aesthetics if .lower or .upper are present in the data. This allows them to work better with automatic orientation detection (and was a bad feature to have existed in the first place anyway). The deprecated tidybayes::geom_intervalh() and tidybayes::geom_pointintervalh() still automatically set those aesthetics, since they are deprecated anyway (so supporting the old behavior is fine in these functions). geom_lineribbon()/stat_lineribbon() now supports a step argument for creating stepped lineribbons. H/T to Solomon Kurz for the suggestion. ggdist now has its own implementation of the scaled and shifted Student's t distribution (dstudent_t(), qstudent_t(), etc), since it is very useful for visualizing confidence distributions