add vanish command

src/util/general.ts

@@ -1,15 +1,16 @@
 import * as readline from "node:readline";
+import type { UserIdResolvable } from "@twurple/api";
 import { apiClient } from "../core/api-client.ts";
 import { logSuccess, logWarning } from "./logger.ts";
 
-export async function getUserId(username: string): Promise<string | null> {
+export async function getUserId(username: string): Promise<UserIdResolvable> {
   const user = await apiClient.users.getUserByName(username);
   if (user?.id) {
     logSuccess(`${user.name} => ${user.id}`);
     return user.id;
   }
   logWarning(`no user with name ${username} found`);
-  return null;
+  return "";
 }
 
 export async function promptForInput(prompt: string): Promise<string> {