feat(GuildPreview): implement support for "preview" endpoint

[SouSinner]

Please describe the changes this PR makes and why it should be merged:
This PR implements a new feature supporting the preview endpoint which is available for public guilds. As mentioned in #3958 — the PR includes the following changes:

• fetchGuildPreview() — method on an instance of Client, used to fetch a public guild if available
• GuildPreview — a structure for an instance of a Guild that's been fetched through the preview endpoint

If anything else needs updating, or if there's been a mistake anywhere, please don't hesitate to point it out; I'll try to resolve it immediately. Also, I'd like to apologize for the former PR that I've requested on my main account, I accidentally pushed changes from my original branch.

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.

[cborac]

Can be more documented

[SouSinner]

I meant
that it's missing parts which should be documented

Could you point the lines of code you believe aren't documented properly? I'll get on it immediately.

[SpaceEEC]

Looking good so far, just a few things:

[kernel-dev]

I've pushed the new changes, hopefully they meet the requirements of what you described, and the methods (and typings) implemented/changed are good now.
However, @SpaceEEC, as you know, I am still conflicted on how to go about the GuildPreview#emojis property. Building a new manager for it seems redundant and unnecessary, however the utility is something we'd like to keep.
Let me know if the way the property is being populated is good, or if there's another way we should go about this. 👍

[advaith1]

Maybe we could also add a fetchPreview() method on Guild as I suggested, for ease of use? like how we have GuildMember.ban() and Guild.members.ban(member)

[kernel-dev]

Sounds good. I'll get on it right now. 👍