Auto merge of #14913 - canaltinova:box-longhands, r=Manishearth

Refactor box longhands

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Converted some longhands into `vector_longhand` to reduce some repeated codes.
Also some of longhands with same `SpecifiedValue` and `computed_value::T` changed. Re-exported SpecifiedValue instead of computed_value::T. Normally a computed_value can't have a parse function.

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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 they're not changing the behavior of the longhands.

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