NavigationObserver register itself in Navigable from constructor and
unregister itself from `finalize()`. The problem is that `finalize()`
won't be invoked for as long as NavigationObserver is visited by
Navigable, leading to GC leaks.
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
