Add support for ggplot2 3.4.0

[barrettk]

Add support for ggplot2 3.4.0 and subsequent versions, while still supporting earlier versions.

Taken from their latest release:

A linewidth aesthetic has been introduced and supersedes the size
aesthetic for scaling the width of lines in line based geoms. size will
remain functioning but deprecated for these geoms and it is recommended to
update all code to reflect the new aesthetic. For geoms that have both point
sizing and linewidth sizing (geom_pointrange() and geom_sf) size now
only refers to sizing of points which can leads to a visual change in old
code (@thomasp85, tidyverse/ggplot2#3672)

In other words, we need to ensure that both size and linewidth attributes are supported for line related _geoms. geom_linerange() was the only affected function pmforest relies on.

closes #26