fix(Invite): fix valueOf returning undefined (#3582)

* Update Invite.js

* Fix typings

* Fix ESLint errors

src/structures/Invite.js

@@ -183,6 +183,10 @@ class Invite extends Base {
       guild: 'guildID',
     });
   }
+
+  valueOf() {
+    return this.code;
+  }
 }
 
 module.exports = Invite;

typings/index.d.ts

@@ -878,6 +878,7 @@ declare module 'discord.js' {
 		public setNickname(nickname: string, reason?: string): Promise<GuildMember>;
 		public toJSON(): object;
 		public toString(): string;
+		public valueOf(): string;
 	}
 
 	export class Integration extends Base {