if the profile provider returns undefined, users will get a confusing error message:

```
No terminal profile options provided for id "nushell_default"
```

if we instead just return a program name "nu", the error message became:

```
path to shell executable "nu" does not exist
```

which should give the user more clue what went wrong.

the error message also guide the user to install `nushell` from website and reload vscode.