feat(decorators): metadata typings added

lib/decorators/telegraf-mention.decorator.ts

@@ -1,12 +1,20 @@
 import { SetMetadata } from '@nestjs/common';
 import { DECORATORS } from '../telegraf.constants';
 
+export type TelegrafMentionUsername = string | string[];
+
+export interface TelegrafMentionMetadata {
+  username: TelegrafMentionUsername;
+}
+
 /**
  * Mention handling.
  * @param username Username
  *
  * https://telegraf.js.org/#/?id=mention
  */
-export function TelegrafMention(username: string | string[]): MethodDecorator {
+export function TelegrafMention(
+  username: TelegrafMentionUsername,
+): MethodDecorator {
   return SetMetadata(DECORATORS.MENTION, { username });
 }