Extends the macOS bell implementation to support the `audio` bell
feature by playing a user-specified audio file via NSSound.
Previously, macOS only supported the `system` feature (NSSound.beep()).
This change adds support for:
- `audio` bell feature: plays the file at `bell-audio-path` using
NSSound, respecting the `bell-audio-volume` setting
- Adds `cval()` to the `Path` type so it can be returned via the C API
Also removes the "(GTK only)" restriction from `bell-audio-path` and
`bell-audio-volume` documentation, as these options now work on macOS.
Example config:
bell-features = audio
bell-audio-path = /System/Library/Sounds/Glass.aiff
bell-audio-volume = 0.8
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