Auto merge of #14049 - Wafflespeanut:lon, r=SimonSapin

Add a generic type for sharing some CSS types

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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 :smile:

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors

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- [x] These changes do not require tests because it's a refactor

r? @SimonSapin (cc @Manishearth @emilio)

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