add &Root<T> checker
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] These changes fix#11137 (github issue number if applicable).
Either:
- [X] There are tests for these changes OR
- [ ] These changes do not require tests because _____
Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process.
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tidy check for vim and emacs modelines
See #10719.
It skips *.webidl files for now since I am not sure where they come from and if they should be edited in tree or not.
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