external completer: support style (#11442)

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# Description

Adds style support.

Reverse highlighting is recommended for the
[completion-menu](https://www.nushell.sh/book/line_editor.html#completion-menu):
```nushell
style: {
    text: white
    selected_text: {
        attr: r
    }
    description_text: white_dimmed
}
```

needs https://github.com/nushell/reedline/pull/691
related https://github.com/nushell/nushell/issues/5292
fix https://github.com/rsteube/carapace/issues/967

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# User-Facing Changes

Style support for values during completion:


![image](https://github.com/nushell/nushell/assets/9090290/f5e4440e-61db-4eeb-87b3-f887b6918c50)


![image](https://github.com/nushell/nushell/assets/9090290/e59065a6-58c2-4f5f-82ea-e2b9c2464b9a)

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crates/nu-cli/src/completions/completer.rs

@@ -2,6 +2,7 @@ use crate::completions::{
     CommandCompletion, Completer, CompletionOptions, CustomCompletion, DirectoryCompletion,
     DotNuCompletion, FileCompletion, FlagCompletion, VariableCompletion,
 };
+use nu_color_config::{color_record_to_nustyle, lookup_ansi_color_style};
 use nu_engine::eval_block;
 use nu_parser::{flatten_expression, parse, FlatShape};
 use nu_protocol::{
@@ -508,6 +509,16 @@ pub fn map_value_completions<'a>(
                         suggestion.description = Some(desc_str);
                     }
                 }
+
+                // Match `style` column
+                if it.0 == "style" {
+                    // Convert the value to string
+                    suggestion.style = match it.1 {
+                        Value::String { val, .. } => Some(lookup_ansi_color_style(val)),
+                        Value::Record { .. } => Some(color_record_to_nustyle(it.1)),
+                        _ => None,
+                    };
+                }
             });
 
             return Some(suggestion);

crates/nu-utils/src/sample_config/default_config.nu

@@ -265,7 +265,7 @@ $env.config = {
             }
             style: {
                 text: green
-                selected_text: green_reverse
+                selected_text: {attr: r}
                 description_text: yellow
             }
         }