Feat/add is username validator

[Moses-Bassey]

Description

Checklist

• [ x] Added username validators that supports just some special strings
• [ x] Also enable users to define thier username string rules

[Clashsoft]

Is this really that useful? It can be done with a regex validator already, and is highly subjective.

[Setitch]

Is this really that useful? It can be done with a regex validator already, and is highly subjective.

Totally agree, not only this adds not needed feature it also strips local language letters unless you provide them - it would be just a pain in multi language environment and real life examples.
One more note: user name is usually email address (especially with 3rd party login system).