Generate correct bindings for callback interfaces: only create an
interface object when the interface declares constants, and set up
the prototype correctly.
This also lets us tidy up some IDL for these callback interfaces.
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