feat(GuildScheduledEvent): add partial and fetch support

[GradyRoberts]

Please describe the changes this PR makes and why it should be merged:
The documentation specifies that the GuildScheduledEvent class supports partials but it is not fully implemented:

TypeError: guildScheduledEvent.fetch is not a function

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• This PR changes the library's interface (methods or parameters added)

---

Local Testing

console.log(`Received GuildScheduledEventUserAdd`);
if (guildScheduledEvent.partial) {
    console.log("Partial guildScheduledEvent, fetching...");
    await guildScheduledEvent.fetch();
}
console.log(guildScheduledEvent);

Output:

Received GuildScheduledEventUserAdd
Partial guildScheduledEvent, fetching...
GuildScheduledEvent {
  id: '1144080228373770280',
  guildId: '859588661548417086',
  channelId: '859588662606168077',
  creatorId: '163032275074678784',
  name: 'tset',
  description: '',
  scheduledStartTimestamp: 1692842400207,
  scheduledEndTimestamp: null,
  privacyLevel: 2,
  status: 1,
  entityType: 2,
  entityId: null,
  userCount: 1,
  creator: User {
    id: '163032275074678784',
    bot: false,
    system: false,
    flags: UserFlagsBitField { bitfield: 0 },
    username: '.grob',
    globalName: 'grob',
    discriminator: '0',
    avatar: '3bf4e6f77edfa65ed5398b4b16c14ded',
    banner: null,
    accentColor: 1710104,
    avatarDecoration: null
  },
  entityMetadata: { location: null },
  image: null
}

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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟢 Performance	94
🟢 Accessibility	96
🟢 Best practices	100
🟠 SEO	67
🔴 PWA	33

Lighthouse ran on https://discord-js-git-fork-gradyroberts-main-discordjs.vercel.app/

[GradyRoberts]

Closing as duplicate #9802