feat: Speed up `get_color_contrast()` #2931

Job	Run time
R-CMD-check / setup	8s
routine / routine	4m 55s
website / pkgdown	4m 27s
R-CMD-check / macOS-latest (4.4.2)	2m 42s
R-CMD-check / windows-latest (4.4.2)	3m 17s
R-CMD-check / ubuntu-latest (devel)	2m 30s
R-CMD-check / ubuntu-latest (4.4.2)	2m 18s
R-CMD-check / ubuntu-latest (4.3.3)	2m 13s
R-CMD-check / ubuntu-latest (4.2.3)	2m 37s
R-CMD-check / ubuntu-latest (4.1.3)	2m 26s
R-CMD-check / ubuntu-latest (4.0.5)	2m 44s
Deploy to shinyapps.io	0s
	30m 17s

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