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/TextTrackCue.cpp

@@ -13,9 +13,9 @@
 
 namespace Web::HTML {
 
-JS_DEFINE_ALLOCATOR(TextTrackCue);
+GC_DEFINE_ALLOCATOR(TextTrackCue);
 
-TextTrackCue::TextTrackCue(JS::Realm& realm, JS::GCPtr<TextTrack> track)
+TextTrackCue::TextTrackCue(JS::Realm& realm, GC::Ptr<TextTrack> track)
     : DOM::EventTarget(realm)
     , m_track(track)
 {