[FEAT]: Support nushell

[bzm3r]

Is your feature request related to a problem? Please describe.
I would like to use nushell along with whkd.

Describe the solution you'd like
Add nushell under supported terminals here:

whkd/src/whkdrc.rs

Line 18 in cc2cbc9

pub enum Shell {

I can try to make a PR for this.

Describe alternatives you've considered
I have considered using PowerShell alongside nushell, but I had some trouble using both while keeping nushell as the "default". For now, I am going to disable nushell as the default shell (via COMSPEC).

Additional context
n/a

[bzm3r]

So, I have a branch where I have something that is at least ready for testing: https://github.com/bzm3r/whkd/blob/master/src/main.rs#L145

Sadly, there is an issue on nushell's end that causes an error:

Error:   × Nushell launched as a REPL, but STDIN is not a TTY; either launch in a valid terminal or provide arguments to invoke a script!

[LGUG2Z]

Very happy to have this added in once the issue in nutshell is resolved!