Futures can currently be cancelled only by dropping the instance. It would be useful to be able cancel them individually.

As an example, if you write a C++ RAII wrapper around a WGPUFuture (say, myapp::WebGPUFuture<wgpu::ComputePipeline>) and you drop it, there's no way to ensure the callback gets called except to wait for the future. If we had a way to cancel futures like void wgpuInstanceCancelFuture(WGPUFuture)¹, you could do that instead - basically, call the callback immediately and ignore/abort any ongoing work.

Right now we already have the InstanceDropped status, I think we could genericize this to Canceled/Cancelled² and use it for this too.