LibJS: Add the HostEnsureCanAddPrivateElement hook

This hook allows us to reject private elements on certain exotic
objects like the window object in browser.
Note that per the spec we should only call this hook if the host is a
web browser, however because LibJS has no way of knowing whether it is
in a web browser environment we just always call the host hook.

Userland/Libraries/LibJS/Runtime/VM.h

@@ -229,6 +229,7 @@ public:
     Function<void(Function<ThrowCompletionOr<Value>()>, Realm*)> host_enqueue_promise_job;
     Function<JobCallback(FunctionObject&)> host_make_job_callback;
     Function<ThrowCompletionOr<void>(Realm&)> host_ensure_can_compile_strings;
+    Function<ThrowCompletionOr<void>(Object&)> host_ensure_can_add_private_element;
 
 private:
     explicit VM(OwnPtr<CustomData>);