pulseaudio: add target option to control source (microphone) volume

The pulseaudio/slider module had partial support for a "target" config
option but changeVolume() in the audio backend was hardcoded to only
modify sink volume. This adds full source volume control:

- Add AudioTarget enum (Sink/Source) to audio_backend
- Extend changeVolume() overloads with AudioTarget parameter
- Store and use pa_cvolume for source (with mono channel default)
- Add sourceVolumeModifyCb for source volume change confirmation
- Add "target" config option to the pulseaudio module for scroll control
- Document "target" in both pulseaudio and pulseaudio-slider man pages

Usage:
  "pulseaudio/slider#in": { "target": "source" }
  "pulseaudio#in": { "target": "source" }

man/waybar-pulseaudio.5.scd

@@ -135,6 +135,11 @@ Additionally, you can control the volume by scrolling *up* or *down* while the c
 	default: false ++
 	Enables this module to consume all left over space dynamically.
 
+*target*: ++
+	typeof: string ++
+	default: sink ++
+	The audio target to control when scrolling. Can be either `sink` (output/speakers) or `source` (input/microphone).
+
 # FORMAT REPLACEMENTS
 
 *{desc}*: Pulseaudio port's description, for bluetooth it'll be the device name.