Text key justification

[teunbrand]

This PR to the RC aims to fix #5614.

Briefly, the issue at hand is that the draw_key_text() and draw_key_label() functions did not know how to handle character-based justification such as "right" or "outward". This PR fixes that by resolving the justification through compute_just().

As a related change, rotate_just() now also handles character justification.

The reprex from #5614:

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

mycars <- mtcars
mycars$name <- rownames(mycars)

ggplot(mycars, aes(wt, mpg, label = name)) +
  geom_text(aes(size = wt), nudge_x = -0.1, hjust = "right")

^{Created on 2024-01-02 with reprex v2.0.2}

[thomasp85]

LGTM