Add from_continuous helper function for categorical use of continuous colormaps

[jkrumbiegel]

Allows to sample continuous colormaps evenly to use them as categorical palettes without having to specify how many categories there are:

using AlgebraOfGraphics
using CairoMakie

spec = data((; x = 1:10, y = 1:10, z = 'A':'J')) *
    mapping(:x, :y, color = :z) *
    visual(BarPlot)

f = Figure()
fg1 = draw!(f[1, 1], spec, scales(Color = (; palette = [:red, :blue])))
legend!(f[1, 2], fg1)
fg2 = draw!(f[1, 3], spec, scales(Color = (; palette = from_continuous([:red, :blue]))))
legend!(f[1, 4], fg2)
f