Remove @ and subsequent characters in usernames (Patreon)

As of version 11.9.0, new settings have been added to the Naming category of the Other tab:

Remove the @ and subsequent characters from the username.

Some users have an @ suffix added to their name, for example:

- Anmi@画集発売中

- 奥馬@skeb募集中

- TonyG @__tony_g

When this setting is enabled, the result of the `{user}` tag in the naming rule will be truncated from the `@` to become:

- Anmi

- 奥馬

- TonyG