Improve look of `as_forest_plot()`

[ddsjoberg]

The current as_forest_plot() creates plots that look like this:

Kat Hoffman recently posted a vv simple method for creating forest plots using ggplot and patchwork. https://twitter.com/kat_hoffman_/status/1606291747545284608

I think updating this function to use these methods will be better overall.

One thing that is lacking in the new method is the size of the boxes to indicate their weights. But I think this is something that can be incorporated somewhat easily.