Editor: Ensure global styles are rendered for third-party blocks.

This change ensures custom styles for all third-party blocks are rendered on the front end if assets are set to be loaded on a per-block basis.  Additionally, this change includes new unit tests to help prevent a similar bug in the future.

Props scruffian, aristath, poena, wildworks, ajlende, andraganescu, ndiego, gigitux, cbravobernal, ramonopoly, andrewserong, oandregal, hellofromTonya, bernhard-reiter.
Fixes #56915.

git-svn-id: https://develop.svn.wordpress.org/trunk@54703 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/data/themedir1/block-theme/theme.json

@@ -69,6 +69,11 @@
 				"filter": {
 					"duotone": "var(--wp--preset--duotone--custom-duotone)"
 				}
+			},
+			"my/third-party-block": {
+				"color": {
+					"background": "hotpink"
+				}
 			}
 		},
 		"elements": {