LibJS+LibWeb: Make HTML::Script GC-allocated

This allows the garbage collector to keep HTML::Script objects alive and
fixes a bug where a HTMLScriptElement could get GC'd while its code was
executing.

Userland/Libraries/LibWeb/HTML/HTMLScriptElement.h

@@ -77,7 +77,7 @@ private:
 
     Function<void()> m_script_ready_callback;
 
-    RefPtr<Script> m_script;
+    JS::GCPtr<Script> m_script;
 
     Optional<DOM::DocumentLoadEventDelayer> m_document_load_event_delayer;