Add a small GLib-backed helper for command stdout that integrates with the main loop instead of blocking on getline() in a worker thread.
The helper keeps the existing child setup semantics used by Waybar commands, including process groups, parent-death signaling, and WAYBAR_OUTPUT_NAME propagation.
This is the foundation for moving long-running custom commands away from manual poll/read logic in the module itself.
Signed-off-by: Austin Horstman <khaneliman12@gmail.com>
