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/User.js

@@ -211,7 +211,7 @@ class User extends Base {
    */
   async createDM() {
     const { dmChannel } = this;
-    if (dmChannel) return dmChannel;
+    if (dmChannel && !dmChannel.partial) return dmChannel;
     const data = await this.client.api.users(this.client.user.id).channels.post({ data: {
       recipient_id: this.id,
     } });