LibWeb: Always call document.close after document.write

This ensures the HTML parser completes running if it previously stopped
at an insertion point during a call to `document.write`.

Tests/LibWeb/Text/input/HTML/document-write-flush-character-insertions.html

@@ -2,5 +2,6 @@
 <script>
     test(() => {
         document.write("PASS");
+        document.close();
     });
 </script>