RequestList cannot be copied or moved, because m_pending_request_queue
contains lambdas that store pointers to the original RequestList and
completion steps that we don't have a reference to.
Fixes a bunch of WPT regressions and imports the ones that work.
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