theme() supports splicing arguments

[teunbrand]

This PR aims to fix #5542.

Briefly, ... is moved to the first argument of the theme (which is technically a breaking change, I suppose), so that it can catch splicing operators. Then find_args(), which does the argument collection, just uses list2() to support the splicing.

Reprex from #5542:

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2

red_axis <- list(
  axis.line = element_line(colour = "red"),
  axis.text = element_text(colour = "red"),
  axis.ticks = element_line(colour = "red"),
  axis.title = element_text(colour = "red")
)

ggplot(mpg, aes(displ, hwy)) +
  geom_point() +
  theme(!!!red_axis)

^{Created on 2023-11-28 with reprex v2.0.2}

[thomasp85]

I cannot imagine anyone using theme() with position matching of args so this should be safe

[teunbrand]

Thanks for the review Thomas!