fix: allow `translationdomain` attr on `<inkscape-extension>`

What does the merge request do?

fix validation schema according to the docs https://inkscape.gitlab.io/extensions/documentation/authors/inx-overview.html#third-party-extensions

• allow translationdomain attr on <inkscape-extension> element
• allow translatable attr on <page> element

Relates:

• https://gitlab.com/inkscape/extras/extension-xaml/-/jobs/4361431980

Checklist

• Add unit tests (if applicable)
• Changes to inkex/ are well documented
• Clean merge request history