This is a bit under-specced, specifically there's no definition of
CSSMarginDescriptors so I've gone with CSSStyleProperties for now. Gets
us 17 WPT subtests.
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