[DomCrawler] Added node name getter

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#11426)
| Applies to    | all
| Fixed tickets |
Makes sense to clarify if it's not 100% clear. However your example is not correct. The filter returns all children of `body` so it will return the HTML tag name of the first element under `body`.

To clear the confusion, how about this:

Access the node name (HTML tag name) of the first node of the current selection (eg. "p" or "div"):
```php
// will return the node name (HTML tag name) of the first child element under <body>
$tag = $crawler->filterXPath('//body/*')->nodeName();
```