Replace AudioStreamPlayer mix target with output channels

[Joseph-DiGiovanni]

This PR replaces the limiting mix_target options of AudioStreamPlayer with more granular output_channels. For example, it is currently not possible to output to only the rear speakers, or only one side. This change allows audio to be output to any combination of speakers, or even just a single one.

In addition, I've added a downmix option which will take the desired output channels and use a standard downmixing equation to best preserve positional audio when using an output device with fewer channels. This is enabled by default, but can be disabled to only play audio when the desired channel is available. A good use case for disabling it would be a speaker test.

While a majority of the work is done, I need some assistance properly removing mix_target in favor of output_channels to not break existing projects. The conversion should be straightforward, but I'm not familiar with that process at all unfortunately.

[Joseph-DiGiovanni]

It seems documentation translations also need to be removed/updated. Any advice for that would be appreciated as well as I'm also unfamiliar with that process.

[AThousandShips]

Translations aren't handled here, but the way this breaks compatibility by removing properties is probably incorrect, and some comaptibility code should be added instead

[fkeyzuwu]

Other than breaking compatibility, I feel like this is displaying too much info for the average user. The solution that first came to mind is adding a "Custom" Mix target, which would only then expose those inspector parameters.

[Joseph-DiGiovanni]

Sorry, I fat fingered the close button 🙃

Can't say I'm opposed to the idea, but I'll have to look into how to implement conditional options.

[Mickeon]

Is there a corresponding proposal for this? If not, I would seriously recommend making one proper. This sounds like it has a very niche use-case.

[Joseph-DiGiovanni]

It may be a niche use case, but to me its more questionable why stereo, center+LFE, and all speakers are the only options. It seems reasonable to expect a way to output to any audio channel and not have arbitrary groups.