[Feature Request] Slash commands

**Is your feature request related to a problem? Please describe.**
Discord recently added slash commands. That is when you just type in `/` and it shows you all kind of operations possible on a discord server.
https://discord.com/developers/docs/interactions/slash-commands

**Describe the solution you'd like**
As a user I want to be able to use slash commands to play, skip or stop music from the music bot

**Describe the basic flow/steps of using this feature**
How this feature would be used:
1. Type `/` into chat or press the slash button
2. See a list of commands available from the music bot
3. Select a command or press "tab" to auto complete it

Start with the Discord slash commands documentation linked in the issue and inspect the repository's existing music-command entry points. Define how play, skip, and stop should appear and behave as slash commands, including command discovery and autocomplete; the issue does not name specific files or tests.

Written by the indexing model from the issue text.