feat: translation linter needs to check numerus format

[patricklodder]

Originally posted by @patricklodder in #3439 (comment)

There's an issue that the linter doesn't catch with numerus: when we delete the numerus form in favor of a single form, the compilation breaks.

In the linter (contrib/devtools/check-translations.py) we should add a check that if there is a numerus="yes" attribute on the <message> element, that its child <translation> element ONLY contains <numerusform> elements, and no text node.

[slightlyskepticalpotat]

Would it be done around here like below?

if translation_node.text is not None:
    print(f'Numerus message contains text {translation_node.text}, needs to be fixed.')
    have_errors = True

[patricklodder]

Would it be done around here like below?

Yes. That is the right place. I'd check for:

1. len(translations) > 0 and generate an error with something like "numerus message doesn't have any numerusform translations"
2. the check you propose, to not have any superfluous text node.