Auto merge of #16522 - chenpighead:stylo-border-reset-border-image, r=heycam

Reset longhands of border-image to their initial value while parsing border shorthand

To do so, we need to declare the border-image longhands as part of border
shorthand and always reset them.

Spec link: https://drafts.csswg.org/css-backgrounds-3/#the-border-shorthands

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