Allow for nanoplot output of horizontal bars

[rich-iannone]

This allows for single line bar plots in cols_nanoplot(). If the plot_type is set to "bar" and single values are found, then horizontal bars will be generated. With single bars, the width is meaningless unless compared against other single values found across other rows. If there is a combination of single values and multiple values within the collection of rows, the single-value bar plots will only be proportional to the other single values (i.e., the different plot views won't interfere with each other).

Here is a simple example:

library(tidyverse)
library(gt)

dplyr::tibble(
  a = c(4.6, 0.2, 5.6, 6.3, 0, NA)
) %>%
  gt() %>%
  cols_nanoplot(columns = a, plot_type = "bar")

This also adds the autoscale arg so that plots can have a shared y and even x scale.