Add wp_natural_time function with unit tests #11750

Job	Run time
Core running from build / Core running from build / Linux	2m 2s
Core running from src / Core running from src / Linux	1m 53s
Core running from build / Core running from build / Windows	3m 43s
Core running from src / Core running from src / Windows	4m 25s
Core running from build / Core running from build / MacOS	2m 42s
Core running from src / Core running from src / MacOS	3m 33s
Gutenberg running from build / Gutenberg running from build / Linux	8m 33s
Gutenberg running from src / Gutenberg running from src / Linux	8m 40s
Gutenberg running from build / Gutenberg running from build / Windows	13m 5s
Gutenberg running from src / Gutenberg running from src / Windows	12m 32s
Gutenberg running from build / Gutenberg running from build / MacOS	10m 1s
Gutenberg running from src / Gutenberg running from src / MacOS	8m 39s
Slack Notifications	0s
Failed workflow tasks	0s
	1h 19m 48s

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