Allow the embedder to register their own VRServices
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Allow the servo embedder to register their own `VRService`s, not just the default ones.
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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 fix#22878
- [X] These changes do not require tests because this is all plumbing
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This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/22886)
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2c63d1296b
