The repository being in static storage is a bit of a hodgepodge, but in
line with how our current storage partitioning is done. We should
eventually move this, along with other across browsing context APIs to a
proper location at a later stage. But for now, this makes progress on
the meat of the BroadcastChannel API.
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
