Use char instead of string for DetectJson (#13019)

* Use char instead of string for DetectJson

* Use faster method

* Change DetectIsJon method

src/Umbraco.Core/Extensions/StringExtensions.cs

@@ -129,8 +129,7 @@ public static class StringExtensions
         }
 
         input = input.Trim();
-        return (input.StartsWith("{") && input.EndsWith("}"))
-               || (input.StartsWith("[") && input.EndsWith("]"));
+        return (input[0] is '[' && input[^1] is ']') || (input[0] is '{' && input[^1] is '}');
     }
 
     public static bool DetectIsEmptyJson(this string input) =>