PHP 8.1

Adds support for

1. Intersection types:

   ->field('countableCollection', intersection(generic(Traversable::class, 'string'), Countable::class))

2. Enum ClassType resolution:

   ->field('enumValue', LookMaIAmEnum::class)

3. Readonly properties:

   ->decorate(new ReadonlyDecorator())

Draft = waits for PHP 8.1 support in PHPStan

cc @jkuchar1 @dkurowski