Make `guide_colourbar()` tick behaviour 'official'

[teunbrand]

This issue flows forth from discussion at #6527.

Briefly, guide_colourbar() interprets a length 2 legend.ticks.length setting as setting for the right/bottom and the left/top separately. Note the asymmetric ticks in the example below.

library(ggplot2)

ggplot(mpg, aes(displ, hwy, colour = cty)) +
  geom_point() +
  theme(
    legend.ticks.length = rel(c(0.5, 0))
  )

^{Created on 2025-09-24 with reprex v2.1.1}

This is an undocumented trick I added in the guide rewrite, but the question is whether we should 'cement' this behaviour to make it official. It'd be a matter of writing down this behaviour in the documentation of guide_colourbar() and adding/amending a test to use this behaviour. We shouldn't document this under the theme(legend.ticks.length) behaviour, because guide_bins() (and most of the legendry package) don't interpret this setting in the described fashion.