Partials (#3070)

* Remove GroupDMChannels

they sparked no joy

* Start partials for message deletion

* MessageUpdate partials

* Add partials as an opt-in client option

* Add fetch() to Message

* Message.author should never be undefined

* Fix channels being the wrong type

* Allow fetching channels

* Refactor and add reaction add partials

* Reaction remove partials

* Check for emoji first

* fix message fetching

janky

* User partials in audit logs

* refactor overwrite code

* guild member partials

* partials as a whitelist

* document GuildMember#fetch

* fix: check whether a structure is a partial, not whether cache is true

* typings: Updated for latest commit (#3075)

* partials: fix messageUpdate behaviour (now "old" message can be partial)

* partials: add warnings and docs

* partials: add partials to index.yml

* partials: tighten "partial" definitions

* partials: fix embed-only messages counting as partials

src/util/Constants.js

@@ -21,6 +21,9 @@ const browser = exports.browser = typeof window !== 'undefined';
  * @property {boolean} [fetchAllMembers=false] Whether to cache all guild members and users upon startup, as well as
  * upon joining a guild (should be avoided whenever possible)
  * @property {boolean} [disableEveryone=false] Default value for {@link MessageOptions#disableEveryone}
+ * @property {PartialType[]} [partials] Structures allowed to be partial. This means events can be emitted even when
+ * they're missing all the data for a particular structure. See the "Partials" topic listed in the sidebar for some
+ * important usage information, as partials require you to put checks in place when handling data.
  * @property {number} [restWsBridgeTimeout=5000] Maximum time permitted between REST responses and their
  * corresponding websocket events
  * @property {number} [restTimeOffset=500] Extra time in millseconds to wait before continuing to make REST
@@ -44,6 +47,7 @@ exports.DefaultOptions = {
   messageSweepInterval: 0,
   fetchAllMembers: false,
   disableEveryone: false,
+  partials: [],
   restWsBridgeTimeout: 5000,
   disabledEvents: [],
   retryLimit: 1,
@@ -261,6 +265,23 @@ exports.Events = {
   RAW: 'raw',
 };
 
+/**
+ * The type of Structure allowed to be a partial:
+ * * USER
+ * * CHANNEL (only affects DMChannels)
+ * * GUILD_MEMBER
+ * * MESSAGE
+ * <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
+ */
+exports.PartialTypes = keyMirror([
+  'USER',
+  'CHANNEL',
+  'GUILD_MEMBER',
+  'MESSAGE',
+]);
+
 /**
  * The type of a websocket message event, e.g. `MESSAGE_CREATE`. Here are the available events:
  * * READY