fix(Typings): remove Partial types from some events

[wasdennnoch]

Please describe the changes this PR makes and why it should be merged:
The typings mark more event arguments as possibly partial than required.

• GUILD_BAN_ADD/GUILD_BAN_ADD always includes a full User object
• GUILD_MEMBER_ADD/GUILD_MEMBERS_CHUNK always includes a full Member object
• USER_UPDATE always includes a full User object
  • GUILD_MEMBER_UPDATE as well, which d.js-internally emits userUpdate

I've tested that the gateway indeed seems to emit full objects for those events.

As the gateway always emits full objects in those cases the d.js-emitted objects will never be partial.

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.