`motion-path`: create a group for later additions

[ddbeck]

#333 covered only the features that aligned with the caniuse motion path feature. There are a number of later additions with a much more complicated compatibility story. We ought to group them and relate them to motion-path in a sensible way.

Affected features:

• css.properties.offset-path.basic-shape
• css.properties.offset-path.coord-box
• css.properties.offset-path.ray
• css.properties.offset-path.url
• css.properties.offset-position
• css.types.ray
• css.types.ray.position
• css.types.ray.size