Theme.json: Fix complex variation selectors when using selectors API #21197

Job	Run time
Determine Matrix	2s
Single Site Default / Test before	14m 42s
Single Site Default / Test current	13m 5s
Multisite Default / Test before	13m 49s
Multisite Default / Test current	13m 10s
Single Site Memcached / Test before	12m 58s
Single Site Memcached / Test current	12m 48s
Multisite Memcached / Test before	3m 49s
Multisite Memcached / Test current	13m 19s
Slack Notifications	0s
${{ matrix.label }}	0s
Failed workflow tasks	0s
	1h 37m 42s

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