Validate username on registration to prevent invalid characters.

[reygcalantaol]

Resolves https://github.com/WPUserManager/wpum-registration-forms/issues/41

Description

Testing Instructions

1. Add username field in the registration form
2. Register a new user with a username that includes invalid character such as +

Pre-review Checklist

• Issue and pull request titles are properly formatted.
• Acceptance criteria have been satisfied and marked in the related issue.
• Unit tests are included (if applicable).
• Self-review of code changes has been completed.
• Self-review of UX changes has been completed.
• Review has been requested from @polevaultweb.