Rendering child fields in field group twig template

If you just need the value of a URL child element in the field group template, access it via

{{ element.field_detail_url[0]['#url'].toString() }}

The "element" variable is available in the templates, even if that is not written down in the field-group-html-element.html.twig comments.

Could this please be prepared as MR? Also needs tests.

Reroll for 3.6 as MR