Categorification: graduate nuon --- from the experimental category to the formats category (#9932)

@jntrnr and I discussed the fact that we can now *graduate* nuon to be a
first class citizen...

This PR moves 

* from nuon
* to nuon

out of the *experimental* stage and into *formats*

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

@@ -39,7 +39,7 @@ impl Command for ToNuon {
                 "specify indentation tab quantity",
                 Some('t'),
             )
-            .category(Category::Experimental)
+            .category(Category::Formats)
     }
 
     fn usage(&self) -> &str {