[css-borders-4] naming of the "not curved" (90deg convex) keyword for corner-shape

[noamr]

Following up on #10993

When the corner shape is a fully convex angle (as in, it's not curved at all, superellipse(infinity)), we resolved on referring to it as none.
I think a better name for this is straight. It's not that the corner has no shape, the corner has a straight shape.

I put it as straight in the draft but we can resolve on this separately if we are ok with it, bring it back to none, or come up with something different.

This keyword is important for animating corner-shape.

[tabatkins]

I agree that using a different keyword than none is better here, now that values are animatable. I think straight is reasonable.