fix(CategoryChannel): set the type to 'category' and document its type

src/structures/CategoryChannel.js

@@ -5,6 +5,10 @@ const GuildChannel = require('./GuildChannel');
  * @extends {GuildChannel}
  */
 class CategoryChannel extends GuildChannel {
+  constructor(guild, data) {
+    super(guild, data);
+    this.type = 'category';
+  }
   /**
    * The channels that are part of this category
    * @type {?Collection<Snowflake, GuildChannel>}

src/structures/Channel.js

@@ -19,6 +19,7 @@ class Channel {
      * * `group` - a Group DM channel
      * * `text` - a guild text channel
      * * `voice` - a guild voice channel
+     * * `category` - a guild category channel
      * @type {string}
      */
     this.type = null;