feat: Speed up get_color_contrast()

[gadenbuie]

Optimizes get_color_contrast() by avoiding re-using bs_get_contrast() and instead directly creating a small Sass layer with a minimal set of Sass files.

This ends up speeding up get_color_contrast() calls by about 25x:

pkgload::load_all()
#> ℹ Loading bslib

results <- bench::mark(
  before = get_color_contrast_old("blue"),
  after = get_color_contrast("blue"), 
  min_time = 2
)

plot(results)

results
#> # A tibble: 2 × 6
#>   expression      min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr> <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 before         29ms  30.57ms      32.3    6.22MB     51.1
#> 2 after         979µs   1.11ms     818.    98.81KB     23.1

[cpsievert]

Nice, thanks!