refactor(Presences): remove from User, nullable on GuildMember

[SpaceEEC]

as well as on Client#presenceUpdate

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

This PR:

• removes User#presence as it was a hack traversing the guild cache in order to find a presence of any member representing the given user.
• makes GuildMember#presence nullable. That way you can distinguish between "This member is offline" and "The presence of this member is unknown" without having to check the cache yourself, rendering this getter pointless.
  Additionally this was confusing when you did not enable the presence intent in the first place: "Why do I know this member is offline? I am not supposed to know!" (Despite you actually having no idea)
• makes Client#presenceUpdate first parameter null instead of undefined, because we don't do this in other places and an explicit null makes more sense than undefined.

Status and versioning classification:

• 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
• This PR changes the library's interface (methods or parameters added)
• This PR includes breaking changes (methods removed or renamed, parameters moved or removed)