LibJS: Hide all the constructors!

Now that the GC allocator is able to invoke Cell subclass constructors
directly via friendship, we no longer need to keep them public. :^)

Userland/Libraries/LibJS/Runtime/Array.h

@@ -44,6 +44,9 @@ public:
 
     [[nodiscard]] bool length_is_writable() const { return m_length_writable; };
 
+protected:
+    explicit Array(Object& prototype);
+
 private:
     ThrowCompletionOr<bool> set_length(PropertyDescriptor const&);