[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.

[jsnkuhn]

Definitely agreed a none shape option doesn't really make sense. There is definitely a shape just a common one.

Going to throw square on the pile as I think it better represents the actual shape of the default corner. I guess technically rectangle might be more correct if the size of the defined corner has a different width than height but square is close enough and it's shorter.

straight makes me think of a straight line which this isn't. It's a 90deg angle.

Wouldn't this shape also be the initial value: #11623