✨ Manual legend: guide_legend_manual()

[teunbrand]

This PR aims to fix #88.

Briefly, it allows the creation of a guide_legend_base() variant where all scale/training is under user control.
@brunomioto as you suggested this in tidyverse/ggplot2#6386, is there any chance I can get you to confirm this would solve the issue?
You can download this PR using: pak::pak("teunbrand/legendry#109")

Example:

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

ggplot(mtcars, aes(disp, mpg)) +
  geom_point() +
  guides(foobar = guide_legend_manual(
    labels = c("foo", "bar"),
    colour = c(NA, "#222222"),
    fill   = c("#d8d8d8", NA),
    layers = geom_col()
  ))

^{Created on 2025-09-28 with reprex v2.1.1}