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/TextTrackCueList.cpp

@@ -12,7 +12,7 @@
 
 namespace Web::HTML {
 
-JS_DEFINE_ALLOCATOR(TextTrackCueList);
+GC_DEFINE_ALLOCATOR(TextTrackCueList);
 
 TextTrackCueList::TextTrackCueList(JS::Realm& realm)
     : DOM::EventTarget(realm, MayInterfereWithIndexedPropertyAccess::Yes)
@@ -58,7 +58,7 @@ size_t TextTrackCueList::length() const
 }
 
 // https://html.spec.whatwg.org/multipage/media.html#dom-texttrackcuelist-getcuebyid
-JS::GCPtr<TextTrackCue> TextTrackCueList::get_cue_by_id(StringView id) const
+GC::Ptr<TextTrackCue> TextTrackCueList::get_cue_by_id(StringView id) const
 {
     // The getCueById(id) method, when called with an argument other than the empty string, must return the first text track cue in the list
     // represented by the TextTrackCueList object whose text track cue identifier is id, if any, or null otherwise. If the argument is the