feat(Partials): add DMChannel/MessageReaction#fetch() and Parti… (#3261)

* add DMChannel#fetch() & Action#getChannel({recipients})

* ref for MessageReaction partial

* typings

* add PartialTypes.REACTION

* accommodate for fully removed reactions

* fix incorrect wording and typo

* typings: MessageReaction#count is nullable

* typings: mark MessageReaction#partial as readonly

Co-Authored-By: Vlad Frangu <kingdgrizzle@gmail.com>

* fix(User): fetch dm channel if cached one is partial

* docs: add missing comma

Co-Authored-By: Antonio Román <kyradiscord@gmail.com>

src/structures/DMChannel.js

@@ -56,7 +56,15 @@ class DMChannel extends Channel {
    * @readonly
    */
   get partial() {
-    return !this.recipient;
+    return this.lastMessageID === undefined;
+  }
+
+  /**
+   * Fetch this DMChannel.
+   * @returns {Promise<DMChannel>}
+   */
+  fetch() {
+    return this.recipient.createDM();
   }
 
   /**