Support Custom modeline

[FlyingWombat]

My display needs custom modeline timings set to work properly (my gpu can't reclock its memory). I'm just checking out Niri, but even if I don't end up using Niri long-term, this will be helpful for others.

An example config could be something like this:

output "DP-1" {
    mode "1332.75 3840 3888 3920 4000 2160 2163 2168 2314 +HSync -VSync"
}