jshn: introduce json_is_a() convenience function to check field types without using a temporary variable

sh/jshn.sh

@@ -244,3 +244,10 @@ json_select() {
 		;;
 	esac
 }
+
+json_is_a() {
+	local type
+
+	json_get_type type "$1"
+	[ "$type" = "$2" ]
+}