Auto merge of #16586 - chenpighead:stylo-moz-border-colors-support, r=heycam

Stylo: add -moz-border-*-colors support

In Gecko, we use double pointers to nsBorderColors to store -moz-border-*-colors.
We can simplify the implementation of computed value in Servo by using Option<Vec>.
As to passing computed values from Servo to Gecko, we might need to use some
binding functions (pre-added in the same Gecko bug, see Bug 1348173).

Note that we also added -moz-border-*-colors as sub_properties of the 'border'
shorthand, so we can make the 'border' shorthand reset -moz-border-*-colors
(See Gecko Bug 482692).

A bit refactoring of replacing all ["top", "right", "bottom", "left"] with PHYSICAL_SIDES
is included in this patch, since we've already had PHYSICAL_SIDES for a while.

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