feat: update username and sign up email should allow username

examples/nextjs-example/lib/auth-client.ts

@@ -5,6 +5,7 @@ import {
 	twoFactorClient,
 	adminClient,
 	multiSessionClient,
+	usernameClient,
 } from "better-auth/client/plugins";
 import { toast } from "sonner";
 
@@ -18,6 +19,7 @@ export const client = createAuthClient({
 		passkeyClient(),
 		adminClient(),
 		multiSessionClient(),
+		usernameClient(),
 	],
 	fetchOptions: {
 		onError(e) {