Consistently use nu as code block lang

[Kissaki]

CONTRIBUTING.md defines a preference of nu over nushell, which overwhelmingly matches our code blocks.

These 66 occurrences were the outliers.

[fdncred]

I think the difference may be that github only recognizes nushell whereas our documentation recognizes nu. Thanks.

[Kissaki]

I wasn't aware that GitHub recognizes nushell; interesting.

I was thinking of the difference between nu as the programming language and interactive REPL examples as well, and whether - if both are supported - it would be worth it to distinguish between the two.

Either way, currently, the docs only use nu. Would be nice if GitHub were to support that too.

[NotTheDr01ds]

@Kissaki thanks for the cleanup/consistency here. Greatly appreciated!

The GitHub problem is apparently that nu is already assigned to an earlier, unrelated language.

In VuePress, we can configure it to use our syntax, but we can't change that on GitHub, I don't believe?

I spent far too long using nu on GitHub and not understanding why it wasn't highlighting properly :-)