Guild/systemchannel (#1799)

* add cool system channel

* Update Guild.js

* Update Guild.js

* Update Guild.js

src/structures/Guild.js

@@ -134,6 +134,12 @@ class Guild {
      */
     this.afkChannelID = data.afk_channel_id;
 
+    /**
+     * The ID of the system channel
+     * @type {?Snowflake}
+     */
+    this.systemChannelID = data.system_channel_id;
+
     /**
      * Whether embedded images are enabled on this guild
      * @type {boolean}
@@ -291,6 +297,24 @@ class Guild {
     return this.members.get(this.ownerID);
   }
 
+  /**
+   * AFK voice channel for this guild
+   * @type {?VoiceChannel}
+   * @readonly
+   */
+  get afkChannel() {
+    return this.client.channels.get(this.afkChannelID);
+  }
+
+  /**
+   * System channel for this guild
+   * @type {?GuildChannel}
+   * @readonly
+   */
+  get systemChannel() {
+    return this.client.channels.get(this.systemChannelID);
+  }
+
   /**
    * If the client is connected to any voice channel in this guild, this will be the relevant VoiceConnection
    * @type {?VoiceConnection}
@@ -525,6 +549,7 @@ class Guild {
     if (data.region) _data.region = data.region;
     if (typeof data.verificationLevel !== 'undefined') _data.verification_level = Number(data.verificationLevel);
     if (data.afkChannel) _data.afk_channel_id = this.client.resolver.resolveChannel(data.afkChannel).id;
+    if (data.systemChannel) _data.system_channel_id = this.client.resolver.resolveChannel(data.systemChannel).id;
     if (data.afkTimeout) _data.afk_timeout = Number(data.afkTimeout);
     if (data.icon) _data.icon = this.client.resolver.resolveBase64(data.icon);
     if (data.owner) _data.owner_id = this.client.resolver.resolveUser(data.owner).id;
@@ -600,6 +625,16 @@ class Guild {
     return this.edit({ afkChannel });
   }
 
+  /**
+   * Edit the system channel of the guild.
+   * @param {ChannelResolvable} systemChannel The new system channel
+   * @param {string} [reason] Reason for changing the guild's system channel
+   * @returns {Promise<Guild>}
+   */
+  setSystemChannel(systemChannel, reason) {
+    return this.edit({ systemChannel }, reason);
+  }
+
   /**
    * Edit the AFK timeout of the guild.
    * @param {number} afkTimeout The time in seconds that a user must be idle to be considered AFK