add command_type to help (#14165)

# Description

This PR adds an indicator when listing subcommands. That indicator tells
whether the command is a plugin, alias, or custom_command.

![image](https://github.com/user-attachments/assets/02889f8a-17b4-4678-bb44-3a487b3d1066)

I changed some of the API to make this work a little easier, namely
`get_signatures()` is now `get_signatures_and_declids()`. It was used in
only one other place (run-external), so I thought it was fine to change
it.

There is a long-standing issue with aliases where they reference the
command name instead of the alias name. This PR doesn't fix that bug.
Example.
```nushell
❯ alias "str fill" = str wrap
```
```nushell
❯ str
... other stuff
Subcommands:

  str wrap (alias) - Alias for `str wrap`
  str wrap (plugin) - Wrap text passed into pipeline.

```


# User-Facing Changes
Slightly different output of subcommands.

crates/nu-protocol/src/engine/engine_state.rs

@@ -820,14 +820,14 @@ impl EngineState {
         }
     }
 
-    /// Get signatures of all commands within scope.
-    pub fn get_signatures(&self, include_hidden: bool) -> Vec<Signature> {
+    /// Get signatures of all commands within scope with their decl ids.
+    pub fn get_signatures_and_declids(&self, include_hidden: bool) -> Vec<(Signature, DeclId)> {
         self.get_decls_sorted(include_hidden)
             .into_iter()
             .map(|(_, id)| {
                 let decl = self.get_decl(id);
 
-                self.get_signature(decl).update_from_command(decl)
+                (self.get_signature(decl).update_from_command(decl), id)
             })
             .collect()
     }