Support NSE args in plt_add()

[grantmcdermott]

Fixes #630

pkgload::load_all("~/Documents/Projects/tinyplot/")
#> ℹ Loading tinyplot
plt(mpg ~ wt, data = mtcars)
plt_add(subset = cyl==4, col = "red", pch = 16)

^{Created on 2026-06-20 with reprex v2.1.1}