Factor out dynamic dotplot binning algorithm

[mjskay]

Based on easystats/see#147 (comment)

While that function looks like a beast, it's really just three main steps: (1) determine bin size, (2) calculate data positions using that bin size, (3) create the grobs. Most likely I will end up with a function for (1) and a function for (2); (3) is pretty straightforward on its own. Hopefully you will then be able to use those two functions to make a custom grob easily.

Plan is something like:

• make a find_dotplot_binwidth() function that figures out the binwidth
  • [ ] (possibly) make the above more generic (i.e. not make assumptions about graphics environment) and have a convenience function like find_dotplot_grob_binwidth() that calls down to the above while figuring out x/y ratios and whatnot from the {grid} environment (on the assumption it is called inside a grob)
• make a bin_dots() function that does the actual binning given a binwidth
• add examples to docs

[mjskay]

Reopening this since I forgot to add examples