Square line end for axis line children

[teunbrand]

This PR also aims to fix #5978 alongside #5981 and is mutually exclusive with #5982.

Briefly, it edits the default theme to have lineend = "square" for the children of axis.line.
The good thing is that it doesn't require any 'hacks' to get it to work:

library(ggplot2)

p <- ggplot(mpg, aes(displ, hwy)) +
  geom_point() +
  theme_classic(base_line_size = 5) +
  theme(axis.ticks.length = unit(5, "mm"))
p

The bad thing is you can't easily swap the lineend, i.e. the following does not work:

p + theme(axis.line = element_line(lineend = "butt"))

Because you'd have to set the lineend in the child elements:

p + theme(
  axis.line.x = element_line(lineend = "butt"),
  axis.line.y = element_line(lineend = "butt")
)

^{Created on 2024-07-08 with reprex v2.1.0}

[teunbrand]

Closing this pending further discussion