Update definition of "animatable" to match WAAPI spec

[gilmoreorless]

When this project was put together, CSS properties were defined in the specs as either animatable as a specific type, or not animatable at all. The advent of the Web Animations API has introduced a change in specs, where "Animatable: no" has been replaced with "Animation type: discrete".

That means this project's definition of "animatable" should actually be "interpolatable", because discrete values are still technically animatable (and the WAAPI defines a distinction between "discrete animation" and "no animation").

This should just be a matter of documentation for now. but it might be worth having different definitions for discrete values (e.g. align-items) vs properties that should never be animated (e.g. display).