Support default values on textareas

[soundmonster]

This PR adds support for default / pre-filled values in <textarea> form elements. The default put_form_data helper function can't work here, as <input> elements have the value in an attribute but <textarea> holds it as a text child.

[soundmonster]

Thank you for flagging this, I was involved in the discussion about deprecating DeepMerge but missed the moment the PR was merged. Will rebase in the PM tomorrow CEST.

[soundmonster]

@germsvel Please have another look:

• rebased on main
• removed DeepMerge
• added textareas to form tests

[germsvel]

Thanks so much @soundmonster! 🥳