Themes: Add "title" key to i18n schema

This is related to Gutenberg changes in WordPress/gutenberg#39936 that marks the "title" key as translatable field in "theme.json" files.

See also #54336.

Props oandregal.





git-svn-id: https://develop.svn.wordpress.org/trunk@53038 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/theme-i18n.json

@@ -1,4 +1,5 @@
 {
+	"title": "Style variation name",
 	"settings": {
 		"typography": {
 				"fontSizes": [