Add strict types to CategoryManager

[core23]

Subject

I am targeting this branch, because this is a minor change.

Changelog

### Added
- Add strict types to CategoryManager

### Changed
- `CategoryManager::getRootCategory` will throw an exception if you pass invalid arguments

[greg0ire]

Maybe avoid the repetition with

private function fromMixedTypeContext(?$context = null): ContextInterface
{
        if (null === $context || \is_string($context)) {
            return $this->getContext($context);
        }

        if ($context instanceof ContextInterface) {
            return $context;
        }

        throw new \InvalidArgumentException(sprintf(
            'Invalid parameter given: %s',
            (string) $context
        ));
}

[greg0ire]

Thanks @core23 !