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/client/actions/MessageReactionAdd.js

@@ -26,11 +26,8 @@ class MessageReactionAdd extends Action {
     if (!message) return false;
 
     // Verify reaction
-    const reaction = message.reactions.add({
-      emoji: data.emoji,
-      count: 0,
-      me: user.id === this.client.user.id,
-    });
+    const reaction = this.getReaction(data, message, user);
+    if (!reaction) return false;
     reaction._add(user);
     /**
      * Emitted whenever a reaction is added to a cached message.