Legend: support title = FALSE

[grantmcdermott]

FIxes #652

pkgload::load_all("~/Documents/Projects/tinyplot/")
#> ℹ Loading tinyplot
plt(mpg ~ wt | factor(am), mtcars, legend = list(title = FALSE, bty = "o"))

^{Created on 2026-06-26 with reprex v2.1.1}

P.S. The existing title = NA behaviour (i.e., no legend but doesn't remove the spacing) is preserved since it follows the base legend() behaviour.