Make `AudioObject+Helpers` extension public

[jolonf]

The getProperty functions in the AudioObject+Helpers.swift file are handy and would only need to be redefined elsewhere if they aren't public.

https://github.com/rnine/SimplyCoreAudio/blob/ba8b651a18a0ad5d10ccc00171bf554fc7c6ae2e/Sources/SimplyCoreAudio/Internal/Extensions/AudioObject%2BHelpers.swift#L13

[ryanfrancesconi]

That seems like it would be a good idea. @rnine ?

[rnine]

Fine by me. However, these functions are not documented, so maybe either they'll need some documentation, or simply use /// :nodoc: so jazzy will ignore them when generating HTML docs. Also, the extension should be moved from Internal/Extensions to Public/Extensions directory to keep intent clear.