Editor: Add plugin template registration API and improve theme overri… #9939

Job	Run time
Core running from build / Core running from build / Linux	2m 18s
Core running from src / Core running from src / Linux	2m 1s
Core running from build / Core running from build / Windows	4m 41s
Core running from src / Core running from src / Windows	4m 11s
Core running from build / Core running from build / MacOS	1m 44s
Core running from src / Core running from src / MacOS	2m 20s
Gutenberg running from build / Gutenberg running from build / Linux	8m 15s
Gutenberg running from src / Gutenberg running from src / Linux	8m 9s
Gutenberg running from build / Gutenberg running from build / Windows	12m 0s
Gutenberg running from src / Gutenberg running from src / Windows	12m 10s
Gutenberg running from build / Gutenberg running from build / MacOS	8m 29s
Gutenberg running from src / Gutenberg running from src / MacOS	6m 54s
Slack Notifications / Prepare notifications	6s
Slack Notifications / Success notifications	4s
Slack Notifications / Cancelled notifications	0s
Slack Notifications / Fixed notifications	0s
Slack Notifications / Failure notifications	0s
Failed workflow tasks	0s
	1h 13m 22s

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