feat: Add Nushell support

[colececil]

Implements #207

Though this implementation includes everything that needs done according to my understanding, it still doesn't seem to be working completely. I'd appreciate some help in figuring out what I might be missing. I believe the environment variables that are passed in to Export are being set correctly in the pre_prompt hook, but I'm not getting the expected SDKs and versions when I try to run them.

Also, note that this implementation wasn't quite as straightforward as I'd hoped, because Nushell doesn't have eval-like functionality. See https://www.nushell.sh/book/how_nushell_code_gets_run.html for more details, and see my comments below that point out how this impacts the implementation.

[bytemain]

Will release 0.7.0 later today

[colececil]

Thanks for reviewing this, @bytemain. Did you have any ideas as to why vfox didn't seem to be working for me in Nushell even with these changes, as I mentioned in my pull request summary above? Were you able to test it out on your machine?

[bytemain]

First of all, I'd like to apologize. I didn't carefully review this PR that morning. I thought the function had already been implemented.
After some research, I've found some methods that can make it work better on NuShell. I'll submit a PR. By the way, NuShell is quite good to use.

[bytemain]

The overall idea is on the right track. but the env -s nushell command yields an empty string (I'm not sure where this command is utilized, and it still returns an empty string even in a Zsh environment). In this case, we need to ensure that env -s nushell returns a valid environment.

[bytemain]

fixed in #397

[colececil]

@bytemain No worries. Thanks for following up on this and getting it to the finish line! I'll take a look at the changes you made in #397 and test them on my Windows machine.