This adds formatters for `WebIDL::Exception`, `WebIDL::SimpleException`
and `WebIDL::DOMException`. These are useful for displaying the content
of errors when debugging.
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