`env` options (similar as hyprland)

[Buer-Nahida]

Discussed in #225

^{Originally posted by SR-MyStar February 24, 2024}
I think niri needs a way to set environment variables in itself config file.

In hyprland, I'm using env = QT_QPA_PLATFORMTHEME, qt5ct to set environment variables for window manager.
In niri, I can write QT_QPA_PLATFORMTHEME=qt5ct in /etc/environment.

But if I do it, these environment variables are play a part in everywhere for my system.
It doesn't make a difference than I set environment variables in window manager config file if I only use wayland.

but I'm not only use niri and other wayland window manager, I also use bspwm(x11 window manager), because some x11 apps are doesn't work use xwayland, they are only work in raw x11 environment, If I write QT_QPA_PLATFORM=wayland in /etc/environment, in bspwm, qt apps are all doesn't work.

So niri needs a way to set environment variables only for itself. such as this

env {
    QT_QPA_PLATFORMTHEME "qt5ct"
}
```</div>

[YaLTeR]

Implemented in ec156a8