[RFC] Support internal function return types

Hi all, RFC: https://wiki.php.net/rfc/internal_function_return_types I noticed that return types RFC[1] didn't support internal functions, I just open a PR[2] to support it. There are some open issues we can discuss: 1. User land return types didn't stop the php, the current implementation raise error to stop PHP from start, this may help debug problem. 2. User land return types didn't support Nullable types, but the RFC return types internally support it, I think it is feature. is that acceptable? Thank you. --- [1] https://wiki.php.net/rfc/return_types [2] https://github.com/php/php-src/pull/1050