feat(Webhook): add support for thread_id in Webhook send

ref: discord/discord-api-docs#2875

src/structures/Webhook.js

@@ -101,6 +101,8 @@ class Webhook {
    * @property {string|boolean} [code] Language for optional codeblock formatting to apply
    * @property {boolean|SplitOptions} [split=false] Whether or not the message should be split into multiple messages if
    * it exceeds the character limit. If an object is provided, these are the options for splitting the message.
+   * @property {Snowflake} [threadID] The id of the thread in the channel to send to.
+   * <info>For interaction webhooks, this property is ignored</info>
    */
 
   /**
@@ -123,6 +125,11 @@ class Webhook {
    *   .then(message => console.log(`Sent message: ${message.content}`))
    *   .catch(console.error);
    * @example
+   * // Send a basic message in a thread
+   * webhook.send('hello!', { threadID: '836856309672348295' })
+   *   .then(message => console.log(`Sent message: ${message.content}`))
+   *   .catch(console.error);
+   * @example
    * // Send a remote file
    * webhook.send({
    *   files: ['https://cdn.discordapp.com/icons/222078108977594368/6e1019b3179d71046e463a75915e7244.png?size=2048']
@@ -173,7 +180,7 @@ class Webhook {
       .post({
         data,
         files,
-        query: { wait: true },
+        query: { thread_id: apiMessage.options.threadID, wait: true },
         auth: false,
       })
       .then(d => {

typings/index.d.ts

@@ -3817,6 +3817,7 @@ declare module 'discord.js' {
     username?: string;
     avatarURL?: string;
     embeds?: (MessageEmbed | object)[];
+    threadID?: Snowflake;
   }
 
   type WebhookTypes = 'Incoming' | 'Channel Follower';