feat: update username and sign up email should allow username

examples/nextjs-example/lib/auth.ts

@@ -7,6 +7,7 @@ import {
 	twoFactor,
 	admin,
 	multiSession,
+	username,
 } from "better-auth/plugins";
 import { reactInvitationEmail } from "./email/invitation";
 import { LibsqlDialect } from "@libsql/kysely-libsql";
@@ -115,5 +116,6 @@ export const auth = betterAuth({
 		bearer(),
 		admin(),
 		multiSession(),
+		username(),
 	],
 });