Implement Slash Commands

[navarr]

https://discord.com/developers/docs/interactions/slash-commands

[McJeffr]

Would be a nice feature, though we'll have to wait for discord.js to support slash commands, which can take a while. According to the maintainers of discord.js, this will be added to v13, which we'll have to update to first before we can look at adding slash commands (when it drops). More info on the PR here: discordjs/discord.js#5106