Editor: add lightbox setting.

Adds lightbox as a valid setting to `WP_Theme_JSON` class and as an image block setting in theme.json.

Props czapla, costdev, desrosj.
Fixes #59393.


git-svn-id: https://develop.svn.wordpress.org/trunk@56643 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/theme.json

@@ -272,6 +272,11 @@
 					"radius": true
 				}
 			},
+			"core/image": {
+				"lightbox": {
+					"allowEditing": true
+				}
+			},
 			"core/pullquote": {
 				"border": {
 					"color": true,