dom: Optimize splitText() (#20371)

This avoids duplicating the intermediate strings, by transferring
ownership.

It's hard to measure the improvement in a reliable way, as we have to
operate on the same node. The following benchmark shows a nice
improvement (although not perfect as a benchmark):
```php
<?php

$dom = new DOMDocument;
$dom->loadXML('<root>testabcdef</root>');
$text = $dom->documentElement->firstChild;

for ($i = 0; $i < 1000000; $i++) {
    $text2 = clone $text;
    $text2->splitText(5);
}
```

Only tested on my desktop i7-4790:
```
Benchmark 1: ./sapi/cli/php x.php
  Time (mean ± σ):     284.1 ms ±   2.8 ms    [User: 280.0 ms, System: 3.0 ms]
  Range (min … max):   281.4 ms … 291.3 ms    10 runs

Benchmark 2: ./sapi/cli/php_old x.php
  Time (mean ± σ):     314.0 ms ±   7.8 ms    [User: 309.2 ms, System: 2.9 ms]
  Range (min … max):   306.5 ms … 328.0 ms    10 runs

Summary
  ./sapi/cli/php x.php ran
    1.11 ± 0.03 times faster than ./sapi/cli/php_old x.php
```

UPGRADING

@@ -117,5 +117,8 @@ PHP 8.6 UPGRADE NOTES
   . Arguments are now passed more efficiently to known constructors (e.g. when
     using new self()).
 
+- DOM:
+  . Made splitText() faster and consume less memory.
+
 - JSON:
   . Improve performance of encoding arrays and objects.

ext/dom/text.c

@@ -127,17 +127,18 @@ PHP_METHOD(DOMText, splitText)
 	first = xmlUTF8Strndup(cur, (int)offset);
 	second = xmlUTF8Strsub(cur, (int)offset, (int)(length - offset));
 
-	xmlNodeSetContent(node, first);
-	nnode = xmlNewDocText(node->doc, second);
-
-	xmlFree(first);
-	xmlFree(second);
+	xmlNodeSetContent(node, NULL);
+	node->content = first;
+	nnode = xmlNewDocText(node->doc, NULL);
 
 	if (nnode == NULL) {
+		xmlFree(second);
 		php_dom_throw_error(INVALID_STATE_ERR, /* strict */ true);
 		RETURN_THROWS();
 	}
 
+	nnode->content = second;
+
 	if (node->parent != NULL) {
 		nnode->type = XML_ELEMENT_NODE;
 		xmlAddNextSibling(node, nnode);