Auto merge of #23675 - asajeffrey:webxr-initialization, r=Manishearth

WebXR initialization

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Initialize a WebXR device registry in Servo.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes do not require tests because this is hidden plumbing at the moment

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