Use phpstan conditional return types to better typehint of returned values.

Ignoring phpcs rule SlevomatCodingStandard.TypeHints.ReturnTypeHint.UselessAnnotation for

/**
 * @return false
 */
public function connectionCloseOk(): bool

Awesome! Thanks for all the typing work you've done! What do you think about adding a types testing file so we are sure all public types work as intended? For example: https://github.com/reactphp-parallel/reactphp-parallel/blob/2.x/tests/Types.php

I would use connectionCloseOk(): false, but it can be used as standalone type only since PHP 8.2.

Planning to do the same as I did with ReactPHP Parallel; keep it at 8.1 for the 0.6 release and then bump it after for the next release.