No partial matching in theme subsetting

[teunbrand]

This PR aims to fix #2724. It supersedes #5335.

Briefly, it implements #2724 (comment) by using .subset2() as the dollar-subsetting method for themes.

Quick demo:

library(ggplot2)

# Old problematic behaviour
t <- theme(foobar = 12)
t$foo
#> [1] 12
t$foobar
#> [1] 12

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

# New correct behaviour
t$foo
#> NULL
t$foobar
#> [1] 12

^{Created on 2023-11-14 with reprex v2.0.2}

[thomasp85]

LGTM