cli: Show beta flags in help output

Looks like

```
   -r --reuse-window         Force to open a file or folder in an already opened window.
 -vvv --verbose              Enable verbose logging.
      --link                 (beta) Securely bind code-server via Coder Cloud with the passed name. You'll get a URL like
                             https://myname.coder-cloud.com at which you can easily access your code-server instance.
                             Authorization is done via GitHub.
```

Based on commits by @JammSpread in #2405

Closes #2396