Add action to regenerate username

The regeneration might be combined with ✨ Update username when updating user entity Needs review by providing a helper method.

@Grevil: Isn't this a duplicate, or am I mistaken?

See my comment here: https://git.drupalcode.org/project/email_registration/-/merge_requests/1...

This is now implemented through ✨ Update username when updating user entity Needs review . My apologies for not properly splitting and moving the code into this issue.