The current css-font-loading spec doesn't define a constructor for this,
and from our own code we can rely on initializing things in the C++
constructor.
For simplicity, this requires that the setlike Foo class has a
`void on_set_modified_from_js(Badge<Bindings::FooPrototype>)` method.
This will be called after the set is modified from a generated `add()`,
`delete()`, or `clear()` method.
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
