Make all properties of `OutputFormat` as `@internal` and to be made `private` in 9.0

[oliverklee]

[JakeQZ]

I've seen @deprecated used for this:

https://github.com/doctrine/instantiator/blob/1.5.x/src/Doctrine/Instantiator/Instantiator.php#L32

    /**
     * ...
     *
     * @deprecated This constant will be private in 2.0
     */

Don't know which is best practice.