Implement wgpuInstanceProcessEvents

[eliemichel]

Polling all devices

[almarklein]

Thanks!

[almarklein]

I happen to look into enabling async support for wgpu-py now, so the timing couldn't be better 😄

@rajveermalviya how does wgpuInstanceProcessEvents (which is in webgpu.h) relate to wgpuDevicePoll (which is in wgpu.h, here)? I presume the latter will be deprecated at some point?

This PR makeswgpuInstanceProcessEvents polls all devices, but is that enough? Does the current call do all the things mentioned here: webgpu-native/webgpu-headers#117?

And then there's webgpu-native/webgpu-headers#199 and webgpu-native/webgpu-headers#314 ... From what I understand, the Future API allows polling for callbacks precisely, while wgpuInstanceProcessEvents is a global "tick" (and does some extra things.