Add a generic type for sharing some CSS types
<!-- Please describe your changes on the following line: -->
This would be useful for types such as `T(pub Option<U>)` or any type that's a mixture of two types (like `LengthOrFoo` for example). Well, it's a bit ugly, especially because we have to address the types as `Either::First` and `Either::Second` everywhere, but I don't have a brighter idea 😄
---
<!-- 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
<!-- Either: -->
- [x] These changes do not require tests because it's a refactor
r? @SimonSapin (cc @Manishearth @emilio)
<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->
<!-- 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/14049)
<!-- Reviewable:end -->