Palette manipulation functions

[teunbrand]

This PR aims to fix #236.

Briefly, it enables the colour manipulation function introduced in #424 to work on palettes introduced in #427.
It also adds a plot() method to palettes to make them easier to visualise.

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

pal_viridis() |>
  col_mix(b = "red") |>
  col_lighter(amount = 30) |>
  plot()

^{Created on 2024-10-10 with reprex v2.1.1}

[thomasp85]

LGTM