Make WebGPU resource creation async and prevent panic during shutdown if WebGPU is enabled.
<!-- Please describe your changes on the following line: -->
1. Make WebGPU resource creation async.
2. Remove some unused code in `WebGPURequest::RequestAdapter`.
3. Prevent panic during shutdown. Since WGPU thread is killed before script, sender and receiver in the script panic at either of the two places-
a. If a buffer is still alive, script tries to send `WebGPURequest::DestroyBuffer` to server while dropping the buffer during shutdown.
7170a69695/components/script/dom/gpubuffer.rs (L118-L122)7170a69695/components/script/dom/gpubuffer.rs (L182-L186)
b. Receiver in script-thread panics with `RecvError` as soon as sender on server side is dropped. 7170a69695/components/script/script_thread.rs (L1456-L1457)
r?@kvark
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix#25472 (GitHub issue number if applicable)
<!-- Either: -->
- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because ___
<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->
<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->