Add isXrCompatible to GPUAdapter

[beaufortfrancois]

Following pattern in https://github.com/gpuweb/gpuweb/blob/main/proposals/compatibility-mode.md#webgpu-spec-changes, I would suggest adding isXrCompatible to GPUAdapter as a convenience to the developer.

This way, the GPU adapter returned withawait navigator.gpu.getAdapter({xrCompatible: true}) will have the isXrCompatible property set to true.

partial interface GPUAdapter {
    readonly attribute boolean isXrCompatible;
}

[AdaRoseCannon]

/tpac to make sure this gets discussed at TPAC

[Yonet]

https://www.w3.org/2024/09/26-immersive-web-minutes.html#t01

[AdaRoseCannon]

Removing the agenda label because I think we discussed this, let me know if that is an error.

[toji]

Reviewed the xrCompatible flag with the WebGPU WG during their October F2F and they were fine adding it to the spec. I'll put up a PR against the WebGPU spec in the coming days.