Legend labeller

[grantmcdermott]

Closes #447

MWE from the updated tinylabel examples:

devtools::load_all("~/Documents/Projects/tinyplot")
#> ℹ Loading tinyplot

s77 = transform(data.frame(state.x77), Illiteracy = Illiteracy / 100)
tinyplot(Life.Exp ~ Income | Illiteracy, data = s77,
         xaxl = '$',
         legend = list(labeller = '%'),
        theme = "clean")

^{Created on 2025-09-20 with reprex v2.1.1}

[grantmcdermott]

Tests passing so merging. I didn't tackle the breaks component since that gets more complicated (e.g., needs to be handled upstream to ensure correct color matching if breaks extend beyond the default range).