add implementation of median.dist_default()

[mjskay]

I ran into a bit of unexpected behavior when making some updates to ggdist. Currently without an implementation of median.dist_default() you can get some errors when trying to get medians of slices of distributions:

> x = dist_normal(1,1)
> median(x)
[1] 1
> x[[1]]
N(1, 1)
> median(x[[1]])
Error in rank(x, ties.method = "min", na.last = "keep") : 
  unimplemented type 'list' in 'greater'

This PR adds an implementation of median.dist_default() that simply copies the implementation of median.distribution() so that the following works:

> x = dist_normal(1,1)
> median(x)
[1] 1
> median(x[[1]])
[1] 1

Happy to adjust as needed.

[mitchelloharawild]

Looks good, thanks. I'll start working toward a release to include these changes.

[mjskay]

Thanks, much appreciated!