* Allow sound instances to play custom audio streams
Adds a new interface FabricSoundInstance, which is injected into
vanilla's SoundInstance interface.
When loading an audio stream, the SoundSystem now calls
FabricSoundInstance.getAudioStream, allowing mods to provide their
own audio streams.
* Some post-review cleanup
- Manually add the client sources as an interface injection source set,
allowing us to put everything in the src/client dir (<3 modmuss50).
- Apply some formatting changes from apple502j.
* Document the empty sound and its usage in sounds.json
* Fix one remaining @literal -> @code
* Fix checkstyle issues
