Add distribution target for CTAN

[alerque]

See for example the difference between a Libertinus release and the CTAN package.

The biggest thing is the included fonts directory should be filtered to just static/OTFS and flattened.

[alerque]

From my last CTAN submission feedback where I asked about format and files to include:

In this case, it was just a matter of moving static/OTF/ and static/WOFF2/ to the top level and and deleted static/. This is not a big deal, so I did it again, and also downcased "OTF/" to "otf/" and "WOFF2/" to "woff2/", because that would seem more usual on CTAN.

I don't know why the WOFF2 were even included because in the same question I mentioned the probably irrelevance of the webfonts to CTAN, but they were included anyway (libertinus-fons).

Also:

Please call the uploaded zipfile "libertinus-fonts.zip" and its top level directory "libertinus-fonts/", like the package id.