Use json_validate in Rules\Json if available (#1394)

[`json_validate` function](https://wiki.php.net/rfc/json_validate)
[added in PHP 8.3](https://php.watch/versions/8.3/json_validate) validates a
given string input to contain valid JSON without decoding it in memory.

This adds a function availability check to `Rules\Json`, and uses the new
function instead of decoding the given input, followed by a last-error check.

library/Rules/Json.php

@@ -36,6 +36,10 @@ final class Json extends AbstractRule
             return false;
         }
 
+        if (function_exists('json_validate')) {
+            return json_validate($input);
+        }
+
         json_decode($input);
 
         return json_last_error() === JSON_ERROR_NONE;