Add `post_title_child_separator` filter #28695

Job	Run time
Core running from src / Windows	3m 41s
Core running from src / MacOS	2m 38s
Core running from build / MacOS	2m 48s
Core running from build / Linux	2m 19s
Core running from src / Linux	2m 15s
Core running from build / Windows	3m 55s
Slack Notifications	0s
Failed workflow tasks	0s
	17m 36s

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