Fix completion on $nu to show correct menus (#8919)

I'm still new to nushell and Rust, so please let me know if there are
any omissions and/or mistakes.

# Description

fixed #8863
`$nu` completion shows wrong completion menus at this time.
This PR fixes the problem to show the correct ones.

# User-Facing Changes

Users can use the correct menus on `$nu` completion like this
[recording](https://asciinema.org/a/KCwfpdAoMFsQODFBnb3NwmufC).

# Tests + Formatting

```
$ use toolkit.nu
$ toolkit check pr

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`
```

# After Submitting

nothing

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

@@ -1,5 +1,5 @@
 use crate::completions::{Completer, CompletionOptions};
-use nu_engine::eval_variable;
+use nu_engine::{column::get_columns, eval_variable};
 use nu_protocol::{
     engine::{EngineState, Stack, StateWorkingSet},
     Span, Value,
@@ -267,7 +267,19 @@ fn nested_suggestions(
 
             output
         }
+        Value::List { vals, span: _ } => {
+            for column_name in get_columns(vals.iter()) {
+                output.push(Suggestion {
+                    value: column_name,
+                    description: None,
+                    extra: None,
+                    span: current_span,
+                    append_whitespace: false,
+                });
+            }
 
+            output
+        }
         _ => output,
     }
 }
@@ -295,6 +307,38 @@ fn recursive_value(val: Value, sublevels: Vec<Vec<u8>>) -> Value {
                     span: Span::unknown(),
                 };
             }
+            Value::LazyRecord { val, span: _ } => {
+                for col in val.column_names() {
+                    if col.as_bytes().to_vec() == next_sublevel {
+                        return recursive_value(
+                            val.get_column_value(col).unwrap_or_default(),
+                            sublevels.into_iter().skip(1).collect(),
+                        );
+                    }
+                }
+
+                // Current sublevel value not found
+                return Value::Nothing {
+                    span: Span::unknown(),
+                };
+            }
+            Value::List { vals, span } => {
+                for col in get_columns(vals.iter()) {
+                    if col.as_bytes().to_vec() == next_sublevel {
+                        return recursive_value(
+                            Value::List { vals, span }
+                                .get_data_by_key(&col)
+                                .unwrap_or_default(),
+                            sublevels.into_iter().skip(1).collect(),
+                        );
+                    }
+                }
+
+                // Current sublevel value not found
+                return Value::Nothing {
+                    span: Span::unknown(),
+                };
+            }
             _ => return val,
         }
     }

crates/nu-cli/tests/completions.rs

@@ -556,6 +556,100 @@ fn variables_completions() {
     // Match results
     match_suggestions(expected, suggestions);
 
+    // Test completions for $nu.os-info
+    let suggestions = completer.complete("$nu.os-info.", 12);
+    assert_eq!(4, suggestions.len());
+    let expected: Vec<String> = vec![
+        "arch".into(),
+        "family".into(),
+        "kernel_version".into(),
+        "name".into(),
+    ];
+    // Match results
+    match_suggestions(expected, suggestions);
+
+    // Test completions for $nu.scope
+    let suggestions = completer.complete("$nu.scope.", 10);
+    assert_eq!(5, suggestions.len());
+    let expected: Vec<String> = vec![
+        "aliases".into(),
+        "commands".into(),
+        "engine_state".into(),
+        "modules".into(),
+        "vars".into(),
+    ];
+    // Match results
+    match_suggestions(expected, suggestions);
+
+    // Test completions for $nu.scope.commands
+    let suggestions = completer.complete("$nu.scope.commands.", 19);
+    assert_eq!(15, suggestions.len());
+    let expected: Vec<String> = vec![
+        "category".into(),
+        "creates_scope".into(),
+        "examples".into(),
+        "extra_usage".into(),
+        "is_builtin".into(),
+        "is_custom".into(),
+        "is_extern".into(),
+        "is_keyword".into(),
+        "is_plugin".into(),
+        "is_sub".into(),
+        "module_name".into(),
+        "name".into(),
+        "search_terms".into(),
+        "signatures".into(),
+        "usage".into(),
+    ];
+    // Match results
+    match_suggestions(expected, suggestions);
+
+    // Test completions for $nu.scope.commands.signatures
+    let suggestions = completer.complete("$nu.scope.commands.signatures.", 30);
+    assert_eq!(17, suggestions.len());
+    let expected: Vec<String> = vec![
+        "any".into(),
+        "binary".into(),
+        "bool".into(),
+        "datetime".into(),
+        "duration".into(),
+        "filesize".into(),
+        "int".into(),
+        "list<any>".into(),
+        "list<binary>".into(),
+        "list<number>".into(),
+        "list<string>".into(),
+        "nothing".into(),
+        "number".into(),
+        "range".into(),
+        "record".into(),
+        "string".into(),
+        "table".into(),
+    ];
+    // Match results
+    match_suggestions(expected, suggestions);
+
+    // Test completions for $nu.scope.engine_state
+    let suggestions = completer.complete("$nu.scope.engine_state.", 23);
+    assert_eq!(6, suggestions.len());
+    let expected: Vec<String> = vec![
+        "num_blocks".into(),
+        "num_decls".into(),
+        "num_env_vars".into(),
+        "num_modules".into(),
+        "num_vars".into(),
+        "source_bytes".into(),
+    ];
+    // Match results
+    match_suggestions(expected, suggestions);
+
+    // Test completions for $nu.scope.vars
+    let suggestions = completer.complete("$nu.scope.vars.", 15);
+    assert_eq!(3, suggestions.len());
+    let expected: Vec<String> = vec!["name".into(), "type".into(), "value".into()];
+    // Match results
+    match_suggestions(expected, suggestions);
+
     // Test completions for custom var
     let suggestions = completer.complete("$actor.", 7);