Having this hidden away in ImageStyleValue meant that
CSSStyleProperties (and anyone else who holds style values) had to know
exactly which types need visiting. This is a footgun waiting to happen,
so make this a virtual method on CSSStyleValue instead.
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
