Nushell support

as nushell is not supported config fails
```bash
❯  wt config shell install
↳ Skipped bash; ~/.bashrc not found
↳ Skipped zsh; ~/.zshrc not found
↳ Skipped fish; ~/.config/fish/functions not found
✗ No shell config files found
```

Nushell has a weird syntax and state POSIX compliance as a non-goal. I also don't know rust so cannot do much beside asking for nushell support :)