KSES: Add support for rgb(a) color. #18049

Job	Run time
Core running from src / Core running from src / MacOS	2m 57s
Core running from src / Core running from src / Linux	2m 17s
Core running from build / Core running from build / MacOS	1m 53s
Core running from build / Core running from build / Linux	2m 36s
Core running from src / Core running from src / Windows	3m 48s
Gutenberg running from src / Gutenberg running from src / MacOS	10m 38s
Core running from build / Core running from build / Windows	4m 50s
Gutenberg running from build / Gutenberg running from build / MacOS	12m 19s
Gutenberg running from src / Gutenberg running from src / Linux	8m 50s
Gutenberg running from build / Gutenberg running from build / Windows	12m 26s
Gutenberg running from src / Gutenberg running from src / Windows	12m 24s
Gutenberg running from build / Gutenberg running from build / Linux	9m 37s
Slack Notifications	0s
Failed workflow tasks	0s
	1h 24m 35s

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