update json for template parts (#3511)

mayland-blocks/experimental-theme.json

@@ -1,4 +1,14 @@
 {
+	"templateParts": [
+		{
+			"name": "header",
+			"area": "header"
+		},
+		{
+			"name": "footer",
+			"area": "footer"
+		}
+	],
 	"settings": {
 		"defaults": {
 			"color": {

seedlet-blocks/experimental-theme.json

@@ -1,12 +1,14 @@
 {
-	"templateParts": {
-		"header": {
+	"templateParts": [
+		{
+			"name": "header",
 			"area": "header"
 		},
-		"footer": {
+		{
+			"name": "footer",
 			"area": "footer"
 		}
-	},
+	],
 	"settings": {
 		"defaults": {
 			"color": {