We already had all necessary things for pseudo elements support in place
except ability to save transition properties in Animatable. This commit
adds the missing part.
`start_needed_transitions()` decides which animations need to be started
based on previous and current style property values. Before this change,
we were using the style value without animations applied as the
"current" value. This caused issues such as starting a new transition
from the animation’s end value when an ongoing animation was
interrupted.
If a property is uses discrete interpolation and TransitionBehavior is
not set to `AllowDiscrete` that property should be non-transitionable.
This is now true for properties whose animation type is not discrete,
but the animation type falls back to discrete.
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
