[deno] Return undefined instead of null from WebGPU APIs

[andyleiserson]

See denoland/deno#29603

The #[undefined] attribute does not seem to work for the async methods GPUBuffer::map_async and GPUQueue::on_submitted_work_done, so those are not fixed.

Testing
The CTS is sensitive to the difference for Queue.submit(), which is why it previously returned undefined manually. No testing for the other functions.

Squash or Rebase? Squash

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.

[ErichDonGubler]

LFG!