[css-animations-2] Define animation-timeline with animation type

[cdoublev]

Replaces Animatable by Animation Type, as defined in Web Animations 1:

How property values combine is defined by the Animation type line in each property’s property definition table

w3c/reffy (spec crawler) does not normalize Animatable to Animation Type therefore users have to check both propDef.animationType and propDef.animatable, which is not great.

[birtles]

Merging without IPR check because it's a purely editorial change.