feat(Guild): nsfw guilds

[almostSouji]

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

As per discord/discord-api-docs#2805 guilds can now be designated as not safe for work. https://support.discord.com/hc/en-us/articles/1500005389362-NSFW-Server-Designation

• At this time PATCH and POST endpoints seem to not support a nsfw property.
• The heavily guarded assignment is used to avoid partials guilds messing up the property, as fix(User): only assign to bot initially or if info is actually present #4990 pointed out for users

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)