add support for cell-paths to NUON (#12718)

# Description
_cell paths_ can be easily serialized back and forth to NUON with the
leading `$.` syntax.

# User-Facing Changes
```nushell
$.foo.bar.0 | to nuon
```
and
```nushell
"$.foo.bar.0" | from nuon
```
are now possible

# Tests + Formatting
a new `cell_path` test has been added to `nuon`

# After Submitting

crates/nuon/src/lib.rs

@@ -14,7 +14,11 @@ pub use to::ToStyle;
 #[cfg(test)]
 mod tests {
     use chrono::DateTime;
-    use nu_protocol::{ast::RangeInclusion, engine::Closure, record, IntRange, Range, Span, Value};
+    use nu_protocol::{
+        ast::{CellPath, PathMember, RangeInclusion},
+        engine::Closure,
+        record, IntRange, Range, Span, Value,
+    };
 
     use crate::{from_nuon, to_nuon, ToStyle};
 
@@ -325,6 +329,20 @@ mod tests {
         );
     }
 
+    #[test]
+    fn cell_path() {
+        nuon_end_to_end(
+            r#"$.foo.bar.0"#,
+            Some(Value::test_cell_path(CellPath {
+                members: vec![
+                    PathMember::string("foo".to_string(), false, Span::new(2, 5)),
+                    PathMember::string("bar".to_string(), false, Span::new(6, 9)),
+                    PathMember::int(0, false, Span::new(10, 11)),
+                ],
+            })),
+        );
+    }
+
     #[test]
     fn does_not_quote_strings_unnecessarily() {
         assert_eq!(