enable block preview (WordPress#62968)

Author: MaggieCabrera

packages/block-library/src/post-content/block.json

@@ -7,6 +7,9 @@
 	"description": "Displays the contents of a post or page.",
 	"textdomain": "default",
 	"usesContext": [ "postId", "postType", "queryId" ],
+	"example": {
+		"viewportWidth": 350
+	},
 	"supports": {
 		"align": [ "wide", "full" ],
 		"html": false,