Implement the missing async functions of WebGPUDriver

Is your feature request related to a problem? Please describe. Implementations of most of the async functions in WebGPUDriver (e.g., WebGPUDriver::createVertexBufferAsyncR) have been skipped so far with only TODO placeholders. We should implement them to advance the WebGPU backend.

Describe the solution you'd like Solution: implement these functions.

Describe alternatives you've considered N/A

OS and backend Is this feature OS/backend specific? If so, explain why. This is specific to the WebGPU backend.