[DomCrawler] Added node name getter

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#11426)
| Applies to    | all
| Fixed tickets |
How about a comment right above this that says what it returns. I actually wasn't 100% sure what "node name" was. I assumed correctly that it was the element name (e.g. `body`) but I wasn't sure!

>
// will return "body"
$tag = $crawler->filterXPath('//body/*')->nodeName();