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/util/Constants.js

@@ -287,6 +287,7 @@ exports.ShardEvents = {
  * * CHANNEL (only affects DMChannels)
  * * GUILD_MEMBER
  * * MESSAGE
+ * * REACTION
  * <warn>Partials require you to put checks in place when handling data, read the Partials topic listed in the
  * sidebar for more information.</warn>
  * @typedef {string} PartialType
@@ -296,6 +297,7 @@ exports.PartialTypes = keyMirror([
   'CHANNEL',
   'GUILD_MEMBER',
   'MESSAGE',
+  'REACTION',
 ]);
 
 /**