is_immutable
Description:
Check whether the given `array` has been marked as immutable by calling [method@Json.
Array.seal] on it.
Parameters:
| this | 
           a JSON array  | 
      
Returns:
| 
           true if the array is immutable  | 
      
Check whether the given `array` has been marked as immutable by calling [method@Json.
Array.seal] on it.
| this | 
           a JSON array  | 
      
| 
           true if the array is immutable  |