Style engine: add CSS var parsing capability to fontSize and fontFamily #364

Job	Run time
Core running from build / Core running from build / Linux	2m 7s
Core running from src / Core running from src / Linux	2m 0s
Core running from build / Core running from build / Windows	3m 33s
Core running from src / Core running from src / Windows	3m 10s
Core running from build / Core running from build / MacOS	5m 5s
Core running from src / Core running from src / MacOS	4m 42s
Gutenberg running from build / Gutenberg running from build / Linux	7m 44s
Gutenberg running from src / Gutenberg running from src / Linux	7m 37s
Gutenberg running from build / Gutenberg running from build / Windows	11m 1s
Gutenberg running from src / Gutenberg running from src / Windows	10m 54s
Gutenberg running from build / Gutenberg running from build / MacOS	11m 16s
Gutenberg running from src / Gutenberg running from src / MacOS	14m 52s
Leave WordPress Playground details	9s
Slack Notifications	0s
Failed workflow tasks	0s
	1h 24m 10s

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