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

Meta/Lagom/Tools/CodeGenerators/LibWeb/GenerateWindowOrWorkerInterfaces.cpp

@@ -64,7 +64,7 @@ static ErrorOr<void> generate_intrinsic_definitions(StringView output_path, Vect
     SourceGenerator generator(builder);
 
     generator.append(R"~~~(
-#include <LibJS/Heap/DeferGC.h>
+#include <LibGC/DeferGC.h>
 #include <LibJS/Runtime/Object.h>
 #include <LibWeb/Bindings/Intrinsics.h>)~~~");