Support `linewidth` aesthetic to control the sizing of the width of lines

ggplot2 recommends using the linewidth aesthetic instead of size as of version 3.4.0 (see this blog post here: https://www.tidyverse.org/blog/2022/11/ggplot2-3-4-0/). The argument in the blog post is as follows: "The reason for this change is that prior to this release size was used for two related, but different, properties: the size of points (and glyphs) and the width of lines. Since one is area based and one is length based they fundamentally needs different scaling and the default size scale has always catered to area sizing, using a square root transform. "

Ideally, the following works in plotnine as well (as it does in ggplot2):

from plotnine import *
from palmerpenguins import load_penguins

penguins = load_penguins().dropna()

(ggplot(penguins, aes(x = "body_mass_g", color = "species"))
   + geom_density(linewidth = 0.75)
)

The issue names the geom_density example but no source file or test path. Start by tracing how plotnine handles the existing size aesthetic for line geoms, then compare the requested linewidth behavior with the linked ggplot2 guidance. Done means the supplied example accepts linewidth=0.75 and uses it to control density line width.