LibGC+Everywhere: Factor out a LibGC from LibJS

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

Libraries/LibWeb/HTML/HTMLOrSVGElement.cpp

@@ -14,7 +14,7 @@ namespace Web::HTML {
 
 // https://html.spec.whatwg.org/multipage/dom.html#dom-dataset-dev
 template<typename ElementBase>
-JS::NonnullGCPtr<DOMStringMap> HTMLOrSVGElement<ElementBase>::dataset()
+GC::Ref<DOMStringMap> HTMLOrSVGElement<ElementBase>::dataset()
 {
     if (!m_dataset)
         m_dataset = DOMStringMap::create(*static_cast<ElementBase*>(this));