[css-properties-values-api] Supports basic shapes / paths

[ewilligers]

<transform-function> is already permitted in syntax strings.

It would be useful to similarly allow path(), circle(), ...

A use case is specifying d and offset-path using an animated custom property, so an element remains on the path while the path is moving. It is currently necessary to animate both properties, and make sure the animations have exactly the same timing and keyframes.