create nuon crate from from nuon and to nuon (#12553)

# Description
playing with the NUON format in Rust code in some plugins, we agreed
with the team it was a great time to create a standalone NUON format to
allow Rust devs to use this Nushell file format.

> **Note**
> this PR almost copy-pastes the code from
`nu_commands/src/formats/from/nuon.rs` and
`nu_commands/src/formats/to/nuon.rs` to `nuon/src/from.rs` and
`nuon/src/to.rs`, with minor tweaks to make then standalone functions,
e.g. remove the rest of the command implementations

### TODO
- [x] add tests
- [x] add documentation

# User-Facing Changes
devs will have access to a new crate, `nuon`, and two functions,
`from_nuon` and `to_nuon`
```rust
from_nuon(
    input: &str,
    span: Option<Span>,
) -> Result<Value, ShellError>
```
```rust
to_nuon(
    input: &Value,
    raw: bool,
    tabs: Option<usize>,
    indent: Option<usize>,
    span: Option<Span>,
) -> Result<String, ShellError>
```

# Tests + Formatting
i've basically taken all the tests from
`crates/nu-command/tests/format_conversions/nuon.rs` and converted them
to use `from_nuon` and `to_nuon` instead of Nushell commands
- i've created a `nuon_end_to_end` to run both conversions with an
optional middle value to check that all is fine

> **Note** 
> the `nuon::tests::read_code_should_fail_rather_than_panic` test does
give different results locally and in the CI...
> i've left it ignored with comments to help future us :)

# After Submitting
mention that in the release notes for sure!!

crates/nu-command/tests/commands/database/into_sqlite.rs

@@ -330,7 +330,7 @@ fn into_sqlite_big_insert() {
                 )
                 .unwrap();
 
-            let nuon = nu_command::value_to_string(&value, Span::unknown(), 0, None).unwrap()
+            let nuon = nuon::to_nuon(&value, true, None, None, Some(Span::unknown())).unwrap()
                 + &line_ending();
 
             nuon_file.write_all(nuon.as_bytes()).unwrap();