Previously, the list was copied when constructing the FormData object,
then the original list was passed to the event, meaning any changes to
the list that happened within the event would not be reflected outside
of it.
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
