Auto merge of #18380 - dadaa:make-font-variation-settings-animatable, r=birtles

Make font-variation-settings animatable

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https://bugzilla.mozilla.org/show_bug.cgi?id=1390702 r=birtles

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- [X] `./mach build -d` does not report any errors
- [X] `./mach build-geckolib` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] `./mach test-tidy --stylo` does not report any errors
- [X] `./mach test-unit -p style` does not report any errors

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- [X] There are tests for these changes. The test codes are patch part 4 in https://bugzilla.mozilla.org/show_bug.cgi?id=1390702

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