Fix username suggestions to only show valid usernames

[milad-emami]

Description

This pull request fixes the issue with username suggestions in the registration form. It ensures that only valid usernames (containing letters, numbers, underscores, and hyphens) are displayed as suggestions, preventing invalid usernames from being suggested, especially for users with Persian or Arabic language settings.

How Has This Been Tested?

I tested the changes by entering various usernames to confirm that only valid suggestions appear and that users can click on them to populate the username field.

[openedx-webhooks]

Thanks for the pull request, @milad-emami!

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[mphilbrick211]

Hi @milad-emami! Just flagging that there are some failing checks here. Please let me know if you have any questions.