Add Guild.acronym

[Soheab]

Summary

This PR adds an acronym property to Guild that returns the characters that show up when a server has no icon set.

The property returns a cached version (_acronym) since it rarely, if ever, changes.

I got this idea from the discord.js library. I have converted it to Python as is.

Discussion post: https://canary.discord.com/channels/336642139381301249/1279035934192439388

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, ...)