Update theme_classic()

[teunbrand]

This PR aims to fix #5978.

It does so by editing the contents of theme_classic(), namely black ticks and axis text and using a square line end for the axis line.
Notably, it does not solve the issue that the 'default' axis line lineend should be square.

Example of theme change, exaggerated linewidth and text size for easier inspection:

library(ggplot2)

ggplot(mpg, aes(displ, hwy)) +
  geom_point() +
  theme_classic(base_line_size = 5) +
  theme(
    axis.ticks.length = unit(5, "mm"),
    axis.text = element_text(size = rel(2))
  )

devtools::load_all("~/packages/ggplot2")
#> ℹ Loading ggplot2

ggplot(mpg, aes(displ, hwy)) +
  geom_point() +
  theme_classic(base_line_size = 5) +
  theme(
    axis.ticks.length = unit(5, "mm"),
    axis.text = element_text(size = rel(2))
  )

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

[thomasp85]

LGTM