BreathingSign direction in a shared staff

[Dan Eble]

I think that the commands that configure single-staff polyphony should set 
Voice.BreathingSign.direction.  Does anyone know of a reason not to change the 
current behavior?

```
\version "2.23.13"

upper = \fixed c'' { c2.-. \breathe d4-. | c2.-. \breathe d4-. }
lower = \fixed c'  { a2.-. \breathe b4-. | a2-.  \breathe d2-. }

\score { << \upper \\ \lower >> }
\score { \partCombine \upper \lower }
```

Thanks,
— 
Dan