[Suggestion] Add nushell support as an exception just like ash in distrobox init

[OJarrisonn]

Is your feature request related to a problem? Please describe.
I'm a nushell user, when creating a distrobox, most distro's package for nushell is called nushell not nu (the name that basename retrieves from $SHELL)

Describe the solution you'd like
Check if the basename is nu then try downloading both nu or nushell or even symlink the nu binary from the host since it uses static linking

Describe alternatives you've considered

Additional context
Some kind of checking is done when using ash i think it won't be too much more code to add this special treatment for nushell