Add application/x-fishscript and application/x-nuscript

The more recent shell fish and nu deserve their mime-type. It will allow DE to recognize them as executable in their UI.

Downstream bug: https://bugs.kde.org/show_bug.cgi?id=473268

Fish reference: https://fishshell.com/docs/current/#shebang-line Nu reference: https://www.nushell.sh/book/scripts.html#shebangs