Enable horizontal margins on (sub)title/caption

[teunbrand]

This PR aims to fix #5533.

As far as I could tell, the (sub)title/captions were the only places where margins were ignored that didn't need to be ignored, so only needed to enable the margins at those places.

Example that margins can be set:

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

ggplot(mpg, aes(displ, hwy)) +
  geom_point() +
  labs(
    title    = "Plot of the mpg dataset",
    subtitle = "Engine displacement versus highway miles per gallon",
    caption  = "EPA fuel economy data"
  ) +
  theme(
    plot.title    = element_text(margin = margin(l = 20, b = 5.5)),
    plot.subtitle = element_text(margin = margin(l = 40, b = 5.5)),
    plot.caption  = element_text(margin = margin(r = 40)) 
  )

^{Created on 2023-11-29 with reprex v2.0.2}

[thomasp85]

LGTM

[teunbrand]

Thanks for the review Thomas!