LibWeb: Flesh out prepare_script and execute_script

This fills in a bunch of the FIXMEs that was in prepare_script.

execute_script is almost finished, it's just missing the module side.

As an aside, let's not assert when inserting a script element with
innerHTML.

Userland/Libraries/LibWeb/HTML/HTMLScriptElement.h

@@ -62,6 +62,13 @@ private:
     bool m_ready_to_be_parser_executed { false };
     bool m_failed_to_load { false };
 
+    enum class ScriptType {
+        Classic,
+        Module
+    };
+
+    ScriptType m_script_type { ScriptType::Classic };
+
     Function<void()> m_script_ready_callback;
 
     String m_script_source;