Sound resource/subresource support

[stashymane]

With sound resources, a pack only has one file to define them, which is sounds.json. It's not possible to add multiple sound resources manually with the provided addAsset, as it overwrites the file instead of appending to it. The only way to work around this at the moment is to regenerate the entire file on modification, which is exponentially not very gucci.

[stashymane]

Essentially the same issue as #37, just more generic