This is a normative change in the ECMA-402 spec. See:
https://github.com/tc39/ecma402/commit/7508197
In our implementation, we don't have the affected AOs directly, as we
delegate to ICU. So instead, we must ensure we provide ICU a locale with
the relevant extension keys present.
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
