[css-borders-4] Allow overriding the stroke style of `border-shape`

[noamr]

See also #13506

With a single-shape border-shape, the shape is stroked.
As with other stroking mechanisms, it is sometimes desirable to define the stroking style - linejoin, linecap etc.
Suggesting to mirror the different stroke-* properties with an equivalent border-shape-* property, e.g. border-shape-linejoin.