Add additional messages to copier.yml

[lyashevska]

E.g. _message_before_copy, _message_after_copy, _message_before_update and _message_after_update. Do not forget to move this line to messages: https://github.com/NLeSC/python-template/blob/7d573c5185aeb7c233e3f67c0436f9383ba59f3e/README.dev.md?plain=1#L76

[fdiblen]

Relevant Copier documentation is here