add "to nuon" enumeration of possible styles (#12591)

# Description
in order to change the style of the _serialized_ NUON data,
`nuon::to_nuon` takes three mutually exclusive arguments, `raw: bool`,
`tabs: Option<usize>` and `indent: Option<usize>` 🤔
this begs to use an enumeration with all possible alternatives, right?

this PR changes the signature of `nuon::to_nuon` to use `nuon::ToStyle`
which has three variants
- `Raw`: no newlines
- `Tabs(n: usize)`: newlines and `n` tabulations as indent
- `Spaces(n: usize)`: newlines and `n` spaces as indent

# User-Facing Changes
the signature of `nuon::to_nuon` changes from
```rust
to_nuon(
    input: &Value,
    raw: bool,
    tabs: Option<usize>,
    indent: Option<usize>,
    span: Option<Span>,
) -> Result<String, ShellError>
```
to
```rust
to_nuon(
    input: &Value,
    style: ToStyle,
    span: Option<Span>
) -> Result<String, ShellError>
```

# Tests + Formatting

# After Submitting

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

@@ -42,14 +42,20 @@ impl Command for ToNuon {
         call: &Call,
         input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
-        let raw = call.has_flag(engine_state, stack, "raw")?;
-        let tabs: Option<usize> = call.get_flag(engine_state, stack, "tabs")?;
-        let indent: Option<usize> = call.get_flag(engine_state, stack, "indent")?;
+        let style = if call.has_flag(engine_state, stack, "raw")? {
+            nuon::ToStyle::Raw
+        } else if let Some(t) = call.get_flag(engine_state, stack, "tabs")? {
+            nuon::ToStyle::Tabs(t)
+        } else if let Some(i) = call.get_flag(engine_state, stack, "indent")? {
+            nuon::ToStyle::Spaces(i)
+        } else {
+            nuon::ToStyle::Raw
+        };
 
         let span = call.head;
         let value = input.into_value(span);
 
-        match nuon::to_nuon(&value, raw, tabs, indent, Some(span)) {
+        match nuon::to_nuon(&value, style, Some(span)) {
             Ok(serde_nuon_string) => {
                 Ok(Value::string(serde_nuon_string, span).into_pipeline_data())
             }