According to the spec, `ResizeObserver` needs to live for as long as
it's referenced from script or has observation targets. With this change
we make sure that `ResizeObserver` is unregistered from the `Document`
when it has no target.
Fixes GC leak that caused us to keep all resize observers alive until
document they belong to is destroyed.
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
