Fix needs_quotes() in to nuon (closes #6989) (#7056)

* to nuon: fix needs_quotes()

Also, null now serialises as null instead of $nothing.

* Clippy

* Add missing quote

* Remove two unnecessary characters

* Add short datetime tests

* Make regex simplificatified

* Alphabetise 'use' statements

* Improve perf by putting case-insensitive cases in regex

* Fix 1 test

crates/nu-command/src/formats/from/nuon.rs

@@ -85,6 +85,7 @@ impl Command for FromNuon {
 
         let mut error = None;
 
+        // Most of the 'work' in converting from Nuon is simply pushing it through the Nu parser.
         let (lexed, err) = nu_parser::lex(string_input.as_bytes(), 0, &[b'\n', b'\r'], &[], true);
         error = error.or(err);