Editor: Fix bug where it was not possible to style custom block elements in theme.json.

This changeset resolves a bug where WordPress would only allow HTML elements within core's own blocks to be styled in `theme.json`. Prior to this change, any `theme.json` rules applying to elements in custom blocks were ignored. With this fix it is now possible to style third-party block elements in `theme.json`.

Props flixos90, azaozz, costdev, glendaviesnz, spacedmonkey, oandregal.
Fixes #57868.


git-svn-id: https://develop.svn.wordpress.org/trunk@56254 602fd350-edb4-49c9-b593-d223f7449a82

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

@@ -73,6 +73,13 @@
 			"my/third-party-block": {
 				"color": {
 					"background": "hotpink"
+				},
+				"elements": {
+					"cite": {
+						"color": {
+							"text": "white"
+						}
+					}
 				}
 			}
 		},