How to limit the word limit of user name during registration?

How to limit the word limit of user name during registration? Such as 16

The username validation can be changed using a local extender in extend.php.

There is a very complete example here https://discuss.flarum.org/d/6507-change-user-validation-rules/9 but the syntax is for an older Flarum version.

This post shows the new syntax https://discuss.flarum.org/d/26815-minimum-password-length/2 but shows how to change the password validation instead.