[css-backgrounds-3] Define properties 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.

Also removes the Animated Values section, which links to CSS Transitions 3 as a future reference defining how values interpolate, but this is now defined in Web Animations 1.