[DomCrawler] Disable CssSelector HTML extension if xml content was added

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | https://github.com/symfony/symfony/issues/8404
| License       | MIT
| Doc PR        |

Don't like the solution too much. Crawler can hold different node types (no restrictions where made to prevent this) but this PR allows disabling HtmlExtension if XML content was added as a string