types(Partials): add toString() method to supported Partials

[janparisek]

Please describe the changes this PR makes and why it should be merged:
Add toString() method to PartialDMChannel, PartialGuildMember and PartialUser in typings.
The following Partials do not support a toString() method and thus remain unchanged:

• PartialMessage, since <PartialMessage>.content is always null
• PartialMessageReaction, since MessageReaction has no toString() method
• PartialThreadMember, since ThreadMember has no toString() method

Fixes #9833

Status and versioning classification:

• I know how to update typings and have done so, or typings don't need updating

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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	87
🟢 Accessibility	96
🟢 Best practices	100
🟠 SEO	67
🔴 PWA	33

Lighthouse ran on https://discord-js-git-fork-janparisek-fixadd-tostring-7db431-discordjs.vercel.app/

[github-actions]

Running Lighthouse audit...

[Jiralite]

Unsure if I'm being too pedantic, but I would type them as ReturnType<Structure["toString"]> since the type will always be the same as the type it comes from.

Would it be possible to add tests too?

[Jiralite]

Just realised that another solution could be this:

export type Partialize<
  T extends AllowedPartial,
  N extends keyof T | null = null,
  M extends keyof T | null = null,
  E extends keyof T | '' = '',
> = {
  [K in keyof Omit<T, E>]: K extends 'partial' ? true : K extends N ? null : K extends M ? T[K] | null : T[K];
};

client and id were always typed as Client<true> and Snowflake respectfully so these could be removed, leaving a simple K extends 'partial' ? true : .... Since there's no intersection with an object, toString() should be fine now. Also fixes the issue where PartialMessageReaction had id typed when it has no id.

[janparisek]

Aight, I'm currently working on it.

TODO:

• Utilize ReturnType<Structure['toString']>
• Write tests
• Refactoring Partialize

[janparisek]

Regarding the tests: I was not sure how many tests would be sufficient and at what point to draw the line. Let me know if I should add checks for additional keys.