Support Slash Commands for the new Discord API

[eibex]

In light of discord.py discontinuation, it will be necessary to rewrite the bot in a new language or library that supports slash commands.

As mentioned in #59 (comment) there are a few discord.py forks that support slash commands. If none seem suitable and stable the rewrite target language will be Rust with the Serenity library.

I am not a Rust developer, it will therefore take time for me to learn the language and migrate the code.

[eibex]

The current candidates are:

• disnake (Support slash commands with Disnake #88)
• enhanced-d.py (Support slash commands with enhanced-discord.py #87)

You are welcome to propose other libraries. The PRs might not be merged until 2022 to see if both libraries continue to receive support. You are free to clone the new branches if you want to try slash commands right away.

[eibex]

I should add that Disnake is currently the better PR, since it includes option choices which I didn't manage to include in edpy.