Add support for nushell

[CRAG666]

I really like this plugin, but I’m currently migrating to nushell and I’d love to have this awesome plugin there.

[carlfriedrich]

You should be able to use forgit in nushell without problems, just put git-forgit anywhere in your PATH and call forgit via subcommands of git like described here. The only thing you have to take care of yourself is defining your aliases, but that's a quite personal choice anyway. If you have done this, feel free to open a PR here in order to bring them upstream.