fixes which showing aliases as built-in nushell commands (#9580)

fixes #8577 

# Description
Currently, using `which` on an alias describes it as a nushell built-in
command:
```bash
> alias foo = print "foo!"                                                                                                                                                                                   > which ls foo --all                                                                                                                                                                                        
╭───┬─────┬──────────────────────────┬──────────╮
│ # │ arg │           path           │ built-in │
├───┼─────┼──────────────────────────┼──────────┤
│ 0 │ ls  │ Nushell built-in command │ true     │
│ 1 │ ls  │ /bin/ls                  │ false    │
│ 2 │ foo │ Nushell built-in command │ true     │
╰───┴─────┴──────────────────────────┴──────────╯
```

This PR fixes the behaviour above to the following:
```bash
> alias foo = print "foo!"
> which ls foo --all
╭───┬─────┬──────────────────────────┬──────────╮
│ # │ arg │           path           │ built-in │
├───┼─────┼──────────────────────────┼──────────┤
│ 0 │ ls  │ Nushell built-in command │ true     │
│ 1 │ ls  │ /bin/ls                  │ false    │
│ 2 │ foo │ Nushell alias            │ false    │
╰───┴─────┴──────────────────────────┴──────────╯
```

# User-Facing Changes
Passing in an alias to `which` will no longer return `Nushell built-in
command`, `true` for `path` and `built-in` respectively.

# Tests + Formatting

# After Submitting

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

@@ -21,6 +21,19 @@ fn which_alias_ls() {
     assert_eq!(actual.out, "Nushell alias: ls -a");
 }
 
+#[test]
+fn which_custom_alias() {
+    let actual = nu!(
+        cwd: ".",
+        r#"alias foo = print "foo!"; which foo | to nuon"#
+    );
+
+    assert_eq!(
+        actual.out,
+        "[[arg, path, built-in]; [foo, \"Nushell alias\", false]]"
+    );
+}
+
 #[test]
 fn which_def_ls() {
     let actual = nu!(