Before and after audio previews

[kmturley]

Currently presets can have an audio preview url, for example:
https://open-audio-stack.github.io/open-audio-stack-registry/presets/jh/floating-rhodes/floating-rhodes.flac
https://github.com/open-audio-stack/open-audio-stack-registry/blob/main/specification.md#preset-schema

This allows users to preview what the sounds will be like with the download. However there is. no way to hear the original instrument. Depending on the instrument selection it could be hard to tell what the preset is actually changing.

Adding a before vs after allows users to toggle the effect on/off to hear the difference.

[RustoMCSpit]

Duplicate #44