Implement PHPStan level 8 compliance for `script-loader.php` #23233

Job	Run time
Gutenberg running from build / Linux	9m 6s
Gutenberg running from src / Linux	8m 54s
Core running from build / Windows	4m 1s
Core running from src / MacOS	3m 12s
Core running from src / Linux	2m 24s
Core running from build / Linux	2m 29s
Gutenberg running from src / Windows	18m 51s
Core running from build / MacOS	2m 21s
Gutenberg running from src / MacOS	9m 49s
Gutenberg running from build / MacOS	13m 47s
Gutenberg running from build / Windows	14m 47s
Core running from src / Windows	3m 49s
Slack Notifications	0s
Failed workflow tasks	0s
	1h 33m 30s

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