`WebIDL::invoke_callback()` now takes an `exception_behavior`
parameter, which can be set to `report` to report an exception before
returning. Setting `exception_behavior` to `rethrow` retains the
previous behavior.
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
