Editor: code quality improvements for theme.json migrate API #6257

Job	Run time
Core running from src / Core running from src / MacOS	1m 32s
Core running from build / Core running from build / MacOS	4m 9s
Gutenberg running from build / Gutenberg running from build / MacOS	8m 50s
Gutenberg running from src / Gutenberg running from src / MacOS	8m 42s
Core running from build / Core running from build / Linux	2m 2s
Core running from src / Core running from src / Linux	1m 55s
Core running from build / Core running from build / Windows	3m 47s
Core running from src / Core running from src / Windows	3m 20s
Gutenberg running from build / Gutenberg running from build / Linux	9m 0s
Gutenberg running from src / Gutenberg running from src / Linux	8m 24s
Gutenberg running from src / Gutenberg running from src / Windows	12m 13s
Gutenberg running from build / Gutenberg running from build / Windows	13m 25s
Slack Notifications	0s
Failed workflow tasks	0s
	1h 17m 19s

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