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.
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