Add AUR to goreleaser

I want stew to automatically be published to the AUR using goreleaser. `Stew` was first released before this was an option in goreleaser.

Docs:
https://goreleaser.com/customization/aur/
https://goreleaser.com/customization/aursources/

Stew in AUR:
https://aur.archlinux.org/packages/stew
https://aur.archlinux.org/packages/stew-bin