estimate_means() : option for prediction interval/include sigma

I'd like to be able to make this plot, visualizing a discrete predictor, with estimated group means and prediction intervals. I'd also like to include densities.

library(see)
dat_mtcars <- transform(mtcars, am = factor(am))
m <- lm(mpg ~ am, data = dat_mtcars)
modelbased::estimate_means(m, at = "am") |> plot() + ggdist::stat_dist_halfeye(ggplot2::aes(x = am, dist = distributional::dist_normal(Mean, sqrt(SE^2 + sigma(m)^2)), fill = am), alpha = .5, width = .2) + see::theme_modern() + ggplot2::guides(fill = "none")

^{Created on 2021-09-08 by the reprex package (v2.0.1)}

To accomplish this, we would need two changes:

1. Add option to estimate_means() for prediction intervals
2. Add option to include densities to estimate means / estimate_prediction family with discrete x

Start with estimate_means() and the estimate_prediction family, using the supplied mtcars and lm() example as the expected use case. Determine how prediction intervals and densities should be exposed for discrete x values; done means both requested options work for the demonstrated plot.