The gist is that we need to construct an ICU date-time formatter for
each possible Temporal type. This is of course going to be expensive.
So instead, we construct the configurations needed for the ICU objects
in the Intl.DateTimeFormat constructor, and defer creating the actual
ICU objects until they are needed.
Each formatting prototype can also now accept either a number (as they
already did), or any of the supported Temporal objects. These types may
not be mixed, and their properties (namely, their calendar) must align
with the Intl.DateTimeFormat object.
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.
