KSES: Add support for rgb(a) color #29613

Job	Run time
Core running from src / Windows	4m 24s
Core running from src / MacOS	2m 45s
Core running from src / Linux	2m 17s
Core running from build / MacOS	2m 14s
Core running from build / Windows	3m 57s
Core running from build / Linux	2m 24s
Slack Notifications	0s
Failed workflow tasks	0s
	18m 1s

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