EZP-22563: Single line break is lost on headers

JIRA: https://jira.ez.no/browse/EZP-22563

# Description

Any `<br>` tag is simply removed if it appears inside an header element and the same goes for line breaks when using the simplified input parser.

This patch adds the support for line breaks inside headers for both the simplified input parser and the Online Editor Input parser while still maintaining the header parsing behavior if there's no line break ie:

```xml
<h1>Franz Ferdinand<br>Love Illumination</h1>
<h1>Right Thoughts, Right Words, Right Action</h1>
```

is converted into (indentation added by me to ease reading):

```xml
<section>
  <header>
    <line xmlns:tmp="http://ez.no/namespaces/ezpublish3/temporary/">Franz Ferdinand</line>
    <line xmlns:tmp="http://ez.no/namespaces/ezpublish3/temporary/">Love Illumination</line>
  </header>
</section>
<section>
  <header>Right Thoughts, Right Words, Right Action</header>
</section>
```

# Tests

manual + unit tests (and the output is correctly formatted in frontend in both legacy and new stacks as well)