This prevents GC leaks caused by a DocumentLoadEventDelayer that is
never destroyed (for example, when its Document becomes inactive) from
holding a strong reference to the Document and keeping it alive.
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