prefs: Make the preferences be guarded by a RwLock instead of a Mutex
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- [x] `./mach test-tidy` does not report any errors
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This should reduce unnecessary locks that happen when we read preferences.
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Add `--version` flag
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy --faster` does not report any errors
- [X] These changes fix#11241 (github issue number if applicable).
Either:
- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____
Not 100% sure of a good way to test this, so I'm submitting as is for feedback. Manually testing it appears to work fine.
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Remove misleading nonexistant flag hint
`./mach build -d` does not report any errors.
`./mach test-tidy --faster` does not report any errors.
The change fixes#11197.
The change does not require tests because it is a small change that doesn't change any core functionality.
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