add PHP Version 8

[mehrshaddarzi]

Please See: https://github.com/wp-cli/wp-cli/blob/da03bb1e8e929fe42a8f9e3814ef284028ee33fc/composer.json#L11

[aaemnnosttv]

Hi there and thanks for the PR. While this would remove the limitation, I want to make sure everything still works as intended with PHP 8 by extending the test matrix to include PHP 8 as well. See #42