Handle legend lazy eval passing

[grantmcdermott]

Fixes #514

pkgload::load_all("~/Documents/Projects/tinyplot")
#> ℹ Loading tinyplot
foo = data.frame(x = 1:2, y = 1:2, grp = c("A", "B"))
class(foo) = c("foo", "data.frame")
tinyplot.foo = function(x, ...) {
  legend = list(title = 'New Title')
  plt(y ~ x | grp, data = x, legend = legend, ...)
}
plt(foo)

^{Created on 2025-11-19 with reprex v2.1.1}