Add support for column and row spans in grid children. #4919

Job	Run time
Core running from build / Core running from build / Linux	2m 3s
Core running from build / Core running from build / Windows	4m 12s
Core running from src / Core running from src / Linux	1m 58s
Core running from src / Core running from src / Windows	5m 19s
Core running from build / Core running from build / MacOS	2m 6s
Core running from src / Core running from src / MacOS	1m 38s
Gutenberg running from build / Gutenberg running from build / Linux	9m 22s
Gutenberg running from src / Gutenberg running from src / Linux	9m 29s
Gutenberg running from build / Gutenberg running from build / Windows	14m 55s
Gutenberg running from src / Gutenberg running from src / Windows	14m 24s
Gutenberg running from build / Gutenberg running from build / MacOS	10m 59s
Gutenberg running from src / Gutenberg running from src / MacOS	10m 58s
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	1h 27m 25s

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