Allow negative values for letter-spacing and word-spacing. Inline Le…
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Allow negative values when parsing `letter-spacing` and `word-spacing`. Inline `parse_non_negative` in `Length`.
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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#15204 (github issue number if applicable).
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- [x] There are tests for these changes
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