<!-- Please describe your changes on the following line: -->
Check that JSON keys are ordered, and that there's no duplicates (for `resources/prefs.json` and `resources/package-prefs.json`).
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<!-- 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] 🆕 `./mach test-tidy --self-test` does not report any errors
- [x] These changes fix#12283
<!-- Either: -->
- [x] There are tests for these changes 🎉
<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->
Source-Repo: https://github.com/servo/servo
Source-Revision: 94d0c485e10138e0d5b2bd8b41b6a922a22a059f
231d5ffcee