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/StorageAPI/NavigatorStorage.cpp

@@ -13,7 +13,7 @@
 namespace Web::StorageAPI {
 
 // https://storage.spec.whatwg.org/#dom-navigatorstorage-storage
-JS::NonnullGCPtr<StorageManager> NavigatorStorage::storage()
+GC::Ref<StorageManager> NavigatorStorage::storage()
 {
     // The storage getter steps are to return this’s relevant settings object’s StorageManager object.
     return HTML::relevant_settings_object(this_navigator_storage_object()).storage_manager();