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/DOM/DocumentLoadEventDelayer.cpp

@@ -10,7 +10,7 @@
 namespace Web::DOM {
 
 DocumentLoadEventDelayer::DocumentLoadEventDelayer(Document& document)
-    : m_document(JS::make_handle(document))
+    : m_document(GC::make_root(document))
 {
     m_document->increment_number_of_things_delaying_the_load_event({});
 }