Add `greetd` as a display manager option

[StuSerious]

Source: https://sr.ht/~kennylevinsen/greetd/ ArchWiki: https://wiki.archlinux.org/title/Greetd

greetd comes with it's own minimal text-based greeter, but also allows for expansion with many other greeters such as tuigreet, mini-wl-greeter, the upcoming cosmic-greeter and many more.

Would love to see this adopted, and I think it would be a great and useful addition, thanks in advance! ~S²