Add a method to fetch a preview of a discoverable guild

[gingershaped]

Summary

This implements a method to fetch a preview of a discoverable guild.

>>> await client.fetch_guild_preview(937435293294919690)
<GuildPreview id=937435293294919690 name='Create Aeronautics' features=['SOUNDBOARD', 'VANITY_URL', 'AUTO_MODERATION', 'PRIVATE_THREADS', 'SEVEN_DAY_THREAD_ARCHIVE', 'THREE_DAY_THREAD_ARCHIVE', 'ROLE_ICONS', 'ANIMATED_ICON', 'GUILD_ONBOARDING_HAS_PROMPTS', 'BANNER', 'GUILD_WEB_PAGE_VANITY_URL', 'GUILD_SERVER_GUIDE', 'CHANNEL_ICON_EMOJIS_GENERATED', 'PREVIEW_ENABLED', 'COMMUNITY', 'AUTOMOD_TRIGGER_USER_PROFILE', 'MEMBER_PROFILES', 'DISCOVERABLE', 'ENABLED_DISCOVERABLE_BEFORE', 'MEMBER_VERIFICATION_GATE_ENABLED', 'GUILD_ONBOARDING', 'NEWS', 'ANIMATED_BANNER', 'INVITE_SPLASH', 'COMMUNITY_EXP_LARGE_GATED', 'GUILD_ONBOARDING_EVER_ENABLED', 'TEXT_IN_VOICE_ENABLED', 'WELCOME_SCREEN_ENABLED'] description='Expanding the Create mod with physics-based contraptions limited only by your own imagination!'>

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

Closes #9890.

[Rapptz]

Hey, thanks for the PR and sorry for the late review.

This PR looks good, it just needs to have the versionchanged strings.