The m_result Variant can hold a GC::Ref<ImportMapParseResult> when the
script element has type="importmap", but visit_edges only traced the
GC::Ref<Script> arm. This left the ImportMapParseResult unvisited,
allowing the GC to collect it while the element still held a reference.
ImportMapParseResult inherited from JS::Script::HostDefined, but no
JS::Script or JS::Module ever stored it as host_defined data, so
visit_host_defined_self was dead code. This removes the HostDefined
inheritance entirely and switches m_result visitation to Variant::visit
with a lambda that catches all GC::Ref arms.
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
The main motivation behind this is to remove JS specifics of the Realm
from the implementation of the Heap.
As a side effect of this change, this is a bit nicer to read than the
previous approach, and in my opinion, also makes it a little more clear
that this method is specific to a JavaScript Realm.
