Add VST3 `preset_data` property to get/set .vstpreset data directly (#351)

* Add VST3 `preset_data` property to get/set .vstpreset data directly

Setting the property has the same behavior as `load_preset` (but
doesn't require a file). Getting the property returns the .vstpreset
data representing the current plugin state.

Previously, it wasn't quite possible to generate .vstpresets purely
from `pedalboard` objects, since they need to include a [class
ID](https://forum.juce.com/t/how-to-get-vst3-class-id-aka-cid-aka-component-id/41041/3),
which isn't exposed elsewhere.

* Fix test formatting