Target the stable webgpu.h headers

[alexreinking]

Our webgpu support is complicated in part because we don't adhere strictly to the stable webgpu.h APIs. The biggest factor here is that we don't really use futures the way they're "meant" to be used. We register callbacks with the event loop and then asynchronously wait for some of them to finish on a clock tick, either via:

• Dawn's special wgpuDeviceTick API
• Emscripten's specific emscripten_sleep API along with setting -s ASYNCIFY=1

I'm not sure why this is necessary. If we had some discipline waiting on futures when we needed their results, we wouldn't run into this. Then we can start tracking webgpu.h evolutions on CI and avoid future breakages.

Related:

#8758
#8757
#8743