Add manual wgpu API #25

This will require some changes to how wgpu-native works, namely we'll definitely want to be able to return Bevy's instance rather than requiring users to create a new instance themselves. libprocessing will need to ship with a defined version of wgpu-native that users will be expected to interact with if they want to share wgpu handles between libprocessing and their custom WebGPU code. There's a ton of trickiness here and this will likely be a pretty difficult task.