Add Nushell

[giggio]

Adding Nushell to the script was thanks to the work done earlier to setup Fish.
I also updated the readme.

I left out the GNU watch comment, as Nushell does not allow for dynamic eval. If we want to support that we would need to generate 2 files, one with the default watch and the other with the GNU tool.

[ahmetb]

I'm inclined to say maybe fish shell was a good place to stop :)

[giggio]

I have the PowerShell one just waiting for this PR. :)
Have you seen how many forks this repo has, adding other shells? It seems to be a common need. It would be nice to have other shells being supported. It would be cool to have this centralized in this repo.
But I understand if you want to keep it more focused, I'll use my fork, then. Feel free to close if that is the direction you want to move.