Unmark DateInterval::$invert as read-only

Despite the PHP documentation recommending that DateInterval properties
should be considered as read-only, there is no way to specify an
inverted interval via the constructor. Therefore, setting this property
on an existing object should not be considered an error.

See: https://www.php.net/manual/en/class.dateinterval.php

stubs/CoreGenericClasses.phpstub

@@ -586,7 +586,6 @@ class DateInterval
     /**
      * Is 1 if the interval is inverted and 0 otherwise
      * @var int
-     * @readonly
      */
     public $invert;