LibWeb: Prepare script when src is set the first time

From https://html.spec.whatwg.org/multipage/scripting.html#script-processing-model:
When a script element el that is not parser-inserted experiences one
of the events listed in the following list, the user agent must
immediately prepare the script element el:
- [...]
- The script element is connected and has a src attribute set where
  previously the element had no such attribute.

Userland/Libraries/LibWeb/HTML/HTMLScriptElement.h

@@ -135,6 +135,8 @@ private:
     Optional<DOM::DocumentLoadEventDelayer> m_document_load_event_delayer;
 
     size_t m_source_line_number { 1 };
+
+    bool m_src_is_set = false;
 };
 
 }