Fix GH-16473: dom_import_simplexml stub is wrong

It's been wrong since PHP 8.0 at least, and the signature was inherited
in 8.4-dev to the new DOM methods.

Author: nielsdos

ext/dom/php_dom.stub.php

@@ -933,4 +933,4 @@ public function registerPhpFunctions(string|array|null $restrict = null): void {
 }
 #endif
 
-function dom_import_simplexml(object $node): DOMElement {}
+function dom_import_simplexml(object $node): DOMAttr|DOMElement {}

ext/dom/php_dom_arginfo.h

@@ -0,0 +1,12 @@
+--TEST--
+GH-16473 (dom_import_simplexml stub is wrong)
+--EXTENSIONS--
+dom
+--FILE--
+<?php
+$root = simplexml_load_string('<root xmlns:x="urn:x" x:attr="foo"/>');
+$spattr = $root->attributes('urn:x');
+var_dump(dom_import_simplexml($spattr)->textContent);
+?>
+--EXPECT--
+string(3) "foo"