Introduce `ReflectionProperty::isUnset()`

Description

There is a difference between uninitialized and unset property when magic method like __get() is defined. The need of these two states was discussed in #9389.

Currently there is no way to detect if property is just uninitialized or also unset, so from the observable state, it is impossible to tell the property behaviour of a given object.

repro: https://3v4l.org/lDeXi

This is a feature request to add ReflectionProperty::isUnset() method.

An alternative would be to update property_exists() function behaviour - https://www.php.net/manual/en/function.property-exists.php#116824 - which does not account for unset property since it was introduced.

リンクされている 3v4l.org の再現例と、php-src にある既存の ReflectionProperty API エントリーポイントから始めてください。未初期化のプロパティと設定解除されたプロパティの動作の違いを定義し、そのうえで ReflectionProperty::isUnset() によってその違いを公開でき、代替となる property_exists() の動作を変更しないことを確認してください。