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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crates/nu-command/tests/commands/alias.rs

@@ -24,7 +24,7 @@ fn alias_hiding_1() {
         cwd: "tests/fixtures/formats", pipeline(
         r#"
             overlay use ./activate-foo.nu;
-            $nu.scope.aliases | find deactivate-foo | length
+            scope aliases | find deactivate-foo | length
         "#
     ));
 
@@ -39,7 +39,7 @@ fn alias_hiding_2() {
         r#"
             overlay use ./activate-foo.nu;
             deactivate-foo;
-            $nu.scope.aliases | find deactivate-foo | length
+            scope aliases | find deactivate-foo | length
         "#
     ));