Swap the alias of Centurii-chan the character and the artist

BUR #35016 is pending approval.

rename centurii-chan -> centurii-chan_(character)
create alias centurii-chan_(artist) -> centurii-chan

For some reason, the self-insert character of Centurii-chan (Artist) does not have an alias (Centurii-chan), but the artist tag itself is. Given that virtually all the artist tag were prioritized over their self-insert/vtuber avatar namesake (e.g. Suzunone Rena over Suzunone Rena (vtuber)), I don't think there's a reason to leave this as an outlier.