feat(Types): support type-guarding using Channel#type string li… (#3918)

* feat(types): union type for Channels

* feat(Types): union for TextBasedChannelTypes

* fix(Types): use new union types in return values

* Update typings/index.d.ts

Co-Authored-By: Sugden <28943913+NotSugden@users.noreply.github.com>

* Update typings/index.d.ts

Co-Authored-By: Sugden <28943913+NotSugden@users.noreply.github.com>

* fix(Types): various minor improvements for Partials

* fix(GuildChannel): partial should return false

Co-authored-by: Sugden <28943913+NotSugden@users.noreply.github.com>
Co-authored-by: Crawl <icrawltogo@gmail.com>

src/structures/GuildChannel.js

@@ -576,6 +576,15 @@ class GuildChannel extends Channel {
     return this.permissionsFor(this.client.user).has(Permissions.FLAGS.MANAGE_CHANNELS, false);
   }
 
+  /**
+   * Whether this GuildChannel is a partial
+   * @type {boolean}
+   * @readonly
+   */
+  get partial() {
+    return false;
+  }
+
   /**
    * Whether the channel is viewable by the client user
    * @type {boolean}