Move Volume and GlobalVolume to own file

[mgi388]

Objective

• Prework for reviving Updated VolumeLevel to use Decibel and Amplitude Ratio Units #9582.

Solution

• Move the two types to volume.rs and made it compile.
• Also #[reflect(Debug)] on Volume while I'm here.

Testing

• Ran example locally.
• Rely on CI.

[alice-i-cecile]

Yes please :)

[SolarLiner]

We should put a pin on improving the documentation of the Volume newtype, because it's unclear from the doc comments which scale we are using (linear can be inferred from the debug_assert!, but it would be nice to make it explicit).

Additionally, we should add a second constructor for setting the volume in decibels.

[mgi388]

We should put a pin on improving the documentation of the Volume newtype, because it's unclear from the doc comments which scale we are using (linear can be inferred from the debug_assert!, but it would be nice to make it explicit).

Additionally, we should add a second constructor for setting the volume in decibels.

Absolutely. We can look at these in a follow up PR since this PR is just a simple move around. FWIW, I also noticed this from a user perspective. The constructor just says Create a new volume level. which leaves the user wondering :)