PHP8 - mixed type support

[SpacePossum]

On master check phpdoc_to_param_type as well.

PhpdocToReturnTypeFixer could support:

            'skip mixed types' => [
                '<?php /** @return Foo|Bar */ function my_foo(): mixed {}',
                '<?php /** @return Foo|Bar */ function my_foo() {}',
            ],

through configuration if we want to