Add @mixin tag

[calebdw]

Hello!

Psalm, PHPStan, and several frameworks use the @mixin tag to delegate unknown calls to other classes:

• https://phpstan.org/writing-php-code/phpdocs-basics#mixins
• https://psalm.dev/docs/annotating_code/supported_annotations/#mixins

It's not specific to Psalm, so I placed it outside of the other Psalm related tags, but just let me know if there's anything you'd like to change.

Thanks!

[claytonrcarter]

Solid. Thank you!