Swaynag is a replacement of i3-nag for sway. Swaynag is embedded in
Sway's build process albeit it is not an integral part of Sway,
therefore it has been added under `wayland.windowManager.sway` instead
of `programs`. It can be moved at a later time if necessary.
Two unit tests were added validate the module behavior for an empty
configuration and the example configuration.
When setting `...sway.package = null`, the default bar configuration
would throw an error trying to use the bar from the null package.
Logic is added to use the bar from `pkgs.sway` instead of `cfg.package`
if it is null.
Fixes#1714
