This is necessary to avoid a circular reference when including
Serializable.h in DOMException.h.
This moves the definition of SerializationRecord, SerializationMemory,
and DeserializationMemory into LibWeb/Forward.h so that Serializable.h
only needs to include LibWeb/Forward.h.
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
