Create an SDL3 audio backend

[bassdr]

This is a follow-up of #1107. For context: We want to see if an SDL3 audio backend would work fine on all platforms, allowing to deprecate other platform-specific audio backends, and potentially keep SDL2 as a fallback.

[Malkierian]

SDL3 audio will have to wait for the full SDL3 conversion, which will happen after SDL_net has a v3+ release.

[bassdr]

I was able to let both SDL2 and SDL3 coexist through dynamic loading the DLLs / so, not ideal but it works.