Includes the following minor functional fixes to deno_webgpu:
* Don't throw an error immediately when `create_buffer` is called with invalid usage flags.
* Implement `on_submitted_work_done`.
* Correct validation of GPUExtent3D element count.
* Run without tracing (instead of panic) if the DENO_WEBGPU_TRACE env var is not set.
Fixes#6838
