Axes internally default to lineend = "square"

[teunbrand]

This PR also aims to fix #5978 alongside #5981 and is mutually exclusive with #5983.

This PR does not edit one of the default themes, but breaks straightforward inheritance of the axis line, but filling in lineend = "square" if lineend hasn't been declared in axis.line.

So, by default we get square lineends:

library(ggplot2)

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

But one can still overrule by setting it explicitly.

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

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

[teunbrand]

Also closing this pending further discussion