split $nu variable into scope commands and simpler $nu (#9487)

# Description

This splits off `scope` from `$nu`, creating a set of `scope` commands
for the various types of scope you might be interested in.

This also simplifies the `$nu` variable a bit.

# User-Facing Changes

This changes `$nu` to be a bit simpler and introduces a set of `scope`
subcommands.

# Tests + Formatting
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- `cargo fmt --all -- --check` to check standard code formatting (`cargo
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- `cargo test --workspace` to check that all tests pass
- `cargo run -- crates/nu-std/tests/run.nu` to run the tests for the
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> from `nushell` you can also use the `toolkit` as follows
> ```bash
> use toolkit.nu # or use an `env_change` hook to activate it
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crates/nu-cli/tests/completions.rs

@@ -538,7 +538,7 @@ fn variables_completions() {
         "loginshell-path".into(),
         "os-info".into(),
         "pid".into(),
-        "scope".into(),
+        "plugin-path".into(),
         "startup-time".into(),
         "temp-path".into(),
     ];
@@ -568,88 +568,6 @@ fn variables_completions() {
     // 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);