[DomCrawler] fixed ticket 7210: DomCrawler is not properly extendable

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #7210
| License       | MIT
| Doc PR        | 

See https://github.com/symfony/symfony/issues/7210 for a description of the argument