Intent to expose WebGPU in early beta Fx139

I intend to enable WebGPU on Windows in the early beta stages of Firefox 139, which enters beta this coming Monday, April 28. "Early beta" extends through Beta 6, so WebGPU would no longer be visible in Beta 7, on May 12. This will help us prepare to let WebGPU ride the trains in Firefox 140, for release on June 24.

WebGPU has been shipped in Chromium. Apple is working on a WebKit implementation for Safari.

WebGPU is currently a W3C Candidate Draft: https://www.w3.org/TR/webgpu.

Mozilla's position on the standard is positive: https://mozilla.github.io/standards-positions/#webgpu

WebGPU has a conformance test suite written using the Web Platform Tests framework: https://github.com/gpuweb/cts

If you would like to try out WebGPU, it has been enabled in Nightly builds for quite some time on all platforms except Android. There are some samples here:

• https://webgpu.github.io/webgpu-samples/

• https://threejs.org/examples/?q=webgpu

On Android, we don't yet have adequate test coverage in CI; however, if you'd like to try it out there anyway, you can set the `gfx.webgpu.ignore-blocklist` pref. There is no pref to enable WebGPU in Release or the present Beta.

Our blockers for shipping WebGPU to release are tracked in Bug 1956070, alias `webgpu-mvp`.