feat(util): add GLib command stream for nonblocking line reads

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>

meson.build

@@ -196,7 +196,8 @@ src_files = files(
     'src/util/regex_collection.cpp',
     'src/util/css_reload_helper.cpp',
     'src/util/transform_8bit_to_rgba.cpp',
-    'src/util/utf8_string.cpp'
+    'src/util/utf8_string.cpp',
+    'src/util/command_line_stream.cpp'
 )
 
 man_files = files(