Feature Request: Rich text for contour labels

[briochemc]

Feature description

I'd like to be able to use rich text for contour labels. Here is a MWE taken from the docs where I just add a labelformatter that simply converts values to strings, but which throws if said formatter uses rich text.

using CairoMakie
himmelblau(x, y) = (x^2 + y - 11)^2 + (x + y^2 - 7)^2
x = y = range(-6, 6; length=100)
z = himmelblau.(x, y')

levels = 10.0.^range(0.3, 3.5; length=10)
colorscale = ReversibleScale(x -> x^(1 / 10), x -> x^10)
labelformatter(z) = map(x -> "$z", z)       # convert to string, works as expected
labelformatter(z) = map(x -> rich("$z"), z) # convert to rich text, throws line below
f, ax, ct = contour(x, y, z; labels=true, labelformatter, levels, colormap=:hsv, colorscale)
f