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

Author: nhooyr

src/node/cli.ts

@@ -197,6 +197,7 @@ const options: Options<Required<Args>> = {
       This is presently beta and requires being accepted for testing.
       See https://github.com/cdr/code-server/discussions/2137
     `,
+    beta: true,
   },
   home: {
     type: "string",