- The `XDG_SESSION_TYPE` environment variable is used by some applications and frameworks to
detect wayland sessions (i.e qt5/6, electron/chromium). It is set by wlroots since version 0.13.0 [1].
- Propagating `XDG_SESSION_TYPE` to the systemd user environment is necessary when processes launched by
services (e.g emacs) need to inherit the environment variable.
[1] - 90c8452959
This option provides a more convenient way to overlay dummy packages.
It also adds a function `config.lib.test.mkStubPackage` that can,
e.g., be used for `package` options.