Use enum BorderWidth instead of a tuple-like struct to store the specified
value. BorderWidth is needed to be used in both longhand and shorthand
border width properties, so I put it in `specified` module.
Fixed#13869.
12902 typedefs
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Using type alias instead of newtype.
Removing duplicated implementation, it's already inherited.
No more type constructors and foo.0
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- [X] These changes fix#12902 (github issue number if applicable).
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