This avoids keeping elements cached in an HTMLCollection alive longer
than necessary in the following scenario:
1. The HTMLCollection cache is populated by querying it.
2. Elements that were included in the cache are removed from the DOM.
3. The cached elements are kept alive by strong references in the cache
until it is updated, which might never happen.
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
