feat: app emojis

[NeloBlivion]

Summary

Implements discord/discord-api-docs#7010
Very rough draft, considering new emoji classes

Information

• 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, typehinting,
  examples, ...).

Checklist

• I have searched the open pull requests for duplicates.
• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why.
• I have updated the changelog to include these changes.

[NeloBlivion]

Removed draft status as this is completely functional, just have to go through and replace instances of Emoji with GuildEmoji (Emoji is still aliased, possibly deprecate for 2.7?)

[Enegg]

Hi, since it came to light that DisnakeDev/disnake#1209 is a blatant copy, I'll share feedback I gave there :)

You should move these two slots to the relevant subclasses, AppEmoji doesn't need a guild_id slot and vice versa

[Lulalaby]

@NeloBlivion can u finish this (see comment)