Update nushell mime type alias to "text/x-nushell"

Recently in a meeting at Nushell it was decided what MIME types we want to recommend. The outcomes are documented here. We don't want to use text/x-nu as it is too short and may come in conflict with other languages. So we recommend using text/x-nushell.

In this MR I just updated the type alias for Nushell as we like the application/x-nuscript primary MIME type.