We now have the Temporal facilities to implement the Date AOs which
parse UTC offset strings using the ISO8601 parser. This patch updates
those AOs and their callers in accordance with the Temporal spec.
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
