ggdist: Visualizations of distributions and uncertainty

New features:

• add pdf and cdf computed variables for the stat_sample_slabinterval() subfamily. See new examples of usage in the last section of vignette("slabinterval"). (#11)
• add cut_cdf_qi() for creating (amongst other things) interval-filled halfeyes, in the style of bayesplot::mcmc_areas() (#11)
• add fill_ramp and color_ramp scales to geom_slabinterval() and geom_lineribbon() families, making it easier to separate group colors from interval/density/CDF colors. See new examples in vignette("slabinterval"), vignette("lineribbon"), and vignette("freq-uncertainty-vis"). (#16)
• auto-detect finite limits on analytical distributions, so (e.g.) distributions like the beta will not cut off tails close to 0 or 1. (#18)
• add brms::brmsprior implementation for parse_dist() (#34)

New documentation:

• vignette("freq-uncertainty-vis") now uses distributional::dist_student_t() (#14)
• add references to fuzzygrams in vignette("slabinterval") (#23)
• add examples with separate positioning of slab and interval (#27)
• add discussion of limitations of curvewise intervals to vignette("lineribbon") (#22)
• soft-deprecate usage of interval_size_range argument in docs (#35)
• initial versions of some cheat sheets

Bug fixes:

• add limited na.rm support to curve_interval() (#22)
• use analytical instead of numerical derivatives on scale transformations where possible, improving reliability.