[css-shapes-1] Simplify the animation type of `shape-outside`

[yarusome]

The animation type of shape-outside is "as defined for <basic-shape>, otherwise discrete", but the clip-path property in a similar situation has its animation type as "by computed value". Web Animations seems to have also covered the case needed by shape-outside in "by computed value", so it may be OK to simplify the animation type of shape-outside as "by computed value".

[astearns]

The "By Computed Value" section in Web Animations doesn’t fit one of the discrete cases for shape-outside: we cannot animate between two shapes from image, so that has to be discrete (even though the computed values have the same number of components and are the same type)