Support of slash commands

[WeeskyBDW]

for as many commands as possible, add the support of Discord's slash commands. But, i suggest to not directly drop the prefix support

idk if discordia support by default slash commands, but community modules exist ([discordia-slash]https://github.com/GitSparTV/discordia-slash for example)

[Mesteery]

idk if discordia support by default slash commands, but community modules exist ([discordia-slash]https://github.com/GitSparTV/discordia-slash for example)

Discordia 3.x supports Slash Commands. And NaB is being updated to Discordia 3.x at https://github.com/DigitalPulseSoftware/NotaBot/tree/discordiav3.