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/src/system/which_.rs

@@ -77,6 +77,8 @@ fn get_entry_in_commands(engine_state: &EngineState, name: &str, span: Span) ->
     if let Some(decl_id) = engine_state.find_decl(name.as_bytes(), &[]) {
         let (msg, is_builtin) = if engine_state.get_decl(decl_id).is_custom_command() {
             ("Nushell custom command", false)
+        } else if engine_state.get_decl(decl_id).is_alias() {
+            ("Nushell alias", false)
         } else {
             ("Nushell built-in command", true)
         };