fix(MessageReaction*Action): correctly cache incoming members and users

[SpaceEEC]

Please describe the changes this PR makes and why it should be merged:

This PR fixes a bug involving partials around the messageReactionAdd event causing it to emit a partial user with an id with the value undefined, as well as caching incoming members and users from said event and typingStart.

The above mentioned bug was caused due to getUser being called with a raw user object, rather than an object with an user_id key, which it actually is expecting.

discord.js/src/client/actions/Action.js

Lines 87 to 90 in b8fd3f6

	getUser(data) {
	const id = data.user_id;
	return data.user || this.getPayload({ id }, this.client.users, id, PartialTypes.USER);
	}

---

MESSAGE_REACTION_ADD, as well as TYPING_START, emit a member and user, which now will be added to the cache, removing the requirement to enable the USER partial type for this event to work on previously uncached users.

If these are missing members and users (due to not originating from a guild), they will be passed long to getUser as before.

Those fields are documented, just not yet implemented by us, see here:
https://discord.com/developers/docs/topics/gateway#message-reaction-add-message-reaction-add-event-fields
https://discord.com/developers/docs/topics/gateway#typing-start-typing-start-event-fields

Status

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.