feat(Channel): add isThread typeguard for better TS support (#5978)

src/structures/Channel.js

@@ -1,7 +1,7 @@
 'use strict';
 
 const Base = require('./Base');
-const { ChannelTypes } = require('../util/Constants');
+const { ChannelTypes, ThreadChannelTypes } = require('../util/Constants');
 const SnowflakeUtil = require('../util/SnowflakeUtil');
 
 /**
@@ -110,6 +110,14 @@ class Channel extends Base {
     return 'messages' in this;
   }
 
+  /**
+   * Indicates whether this channel is a thread channel.
+   * @returns {boolean}
+   */
+  isThread() {
+    return ThreadChannelTypes.includes(this.type);
+  }
+
   static create(client, data, guild) {
     const Structures = require('../util/Structures');
     let channel;

typings/index.d.ts

@@ -438,6 +438,7 @@ declare module 'discord.js' {
     public delete(reason?: string): Promise<Channel>;
     public fetch(force?: boolean): Promise<Channel>;
     public isText(): this is TextChannel | DMChannel | NewsChannel | ThreadChannel;
+    public isThread(): this is ThreadChannel;
     public toString(): string;
   }