This PR was merged into the 5.2-dev branch.

Discussion
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[Serializer] Adds FormErrorNormalizer

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | no
| Tickets       | Fix #27428
| License       | MIT
| Doc PR        | symfony/symfony-docs#13885
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# Readme
Adds `FormErrorNormalizer` for AJAX forms (see the suggestion: #27428). It was on the [FOSRestBundle](https://github.com/FriendsOfSymfony/FOSRestBundle/blob/master/Serializer/Normalizer/FormErrorNormalizer.php) and now it is implemented in the `Serializer` component.

# Roadmap
- [x] Core of the class
- [x] Output syntax
- [x] Support of the children forms
- [x] Doc

Commits
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dcb8d8b [Serializer] Adds FormErrorNormalizer