Split into tidybayes / ggdist (or some other name)

[mjskay]

Move geoms and such into a separate repo then reexport them in tidybayes.

Possibly consider some cleanup in the process (e.g. geom_eye would remain in tidybayes only as a legacy spelling; would do all of this after implementing support for the new orientation-sensing geoms and drop all the h suffix spellings from ggdist, etc)

• [x] split
• [x] add ggdist (or whatever) to https://exts.ggplot2.tidyverse.org/gallery/