style: Make 'font' shorthand reset 'font-variation-settings' property
As required by the spec: https://drafts.csswg.org/css-fonts-4/#font-prop
See https://bugzilla.mozilla.org/show_bug.cgi?id=1435983
Basically, make font-variation-settings work in the same way as font-feature-settings
already does.
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).
- [ ] There are tests for these changes OR
- [X] These changes do not require tests because font-variation-settings isn't supported in servo; it is implemented here for gecko/stylo, and will be tested by mozilla-central mochitests.
<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/20191)
<!-- Reviewable:end -->
1783e41f34
