Named palettes

[teunbrand]

This PR aims to fix #396.

Briefly, it adds a register of known palettes.

In some more detail, you can get one of the known palettes with get_palette(), set a new known palette with set_pallette(), see all known palettes with palette_names() and restore the default palettes with reset_palettes(). Along with these functions, there is integration with as_continuous/discrete_pal() and show_col() to work with known palettes.

The nice thing about having known palettes, is that you can refer to them by name.

devtools::load_all("~/packages/scales/")
#> ℹ Loading scales
show_col("Okabe-Ito")

This PR would nicely integrate with tidyverse/ggplot2#6112, as it would use as_continuous/discrete_pal() in the default colour scales.

pal <- as_discrete_pal("magma")
show_col(pal(16))

Palette packages are then able to register their own palettes, which you could then also refer to by name.

cols <- c("palegreen", "deepskyblue", "magenta")
set_palette("aurora", palette = cols)
show_col(as_continuous_pal("aurora"), labels = FALSE)

^{Created on 2024-09-19 with reprex v2.1.1}