Fix && quotation in to nuon after proptest fail (#7564)

`proptest` caught a failing test condition for `&&` as a literal string. It requires a quotation to be parsed correctly by current `from nuon`
    
https://github.com/nushell/nushell/actions/runs/3753242377/jobs/6376308675

The change in the parser that now returns an error was introduced by https://github.com/nushell/nushell/pull/7241

This in theory doesn't have to be an error (it is a diagnostic for nushell code) but it is probably better safe than sorry to require quotation here.

- Add a test for `&&` in `to nuon` from proptest fail
- Fix `to nuon` generating invalid `&&` literal
- Add a test for `,` in `to nuon`/`from nuon` cycle
  - Bonus: should already be properly quoted

crates/nu-command/tests/format_conversions/nuon.rs

@@ -330,7 +330,9 @@ fn quotes_some_strings_necessarily() {
             '-11.0..-15.0', '11.0..-15.0', '-11.0..15.0',
             '-11.0..<-15.0', '11.0..<-15.0', '-11.0..<15.0',
             '-11.0..', '11.0..', '..15.0', '..-15.0', '..<15.0', '..<-15.0',
-            '2000-01-01', '2022-02-02T14:30:00', '2022-02-02T14:30:00+05:00'
+            '2000-01-01', '2022-02-02T14:30:00', '2022-02-02T14:30:00+05:00',
+            ',',
+            '&&'
             ] | to nuon | from nuon | describe
         "#
     ));