don't return anything from toolkit command (#992)

i get the following LSP error in my editor, because it's true `main`
does not return a `string` 🤔


![image](https://github.com/user-attachments/assets/787f62f8-ef66-4ac4-ba30-c396d3cdd4d2)


in this PR, i propose to truely return `nothing` from this _main_
command by `print`ing the help page.

### alternative
we could also change the signature to `nothing -> string`

toolkit.nu

@@ -35,7 +35,7 @@ export def "check pr" [
 }
 
 # View subcommands.
-export def main []: nothing -> nothing {
+export def main []: nothing -> string {
     help toolkit
 }