Add mime types for Mallard and Ducktype to the shared-mime-info database

Hi,

I've opened the following ticket:

https://gitlab.freedesktop.org/xdg/shared-mime-info/-/issues/184
"Add Mallard and Ducktype mime types"

Would be great to have your opinion/advice/expertise. Note that I've
not yet created a merge request, but I can give it a try in the
following days.

Cheers,
Sébastien