Add sharding support

[PrivateGER]

How have you installed cordless

• Arch User Repository
• Snap
• scoop
• brew
• go get

What happened

Attempting to log in as a bot with a bot that is part of more than 2000 servers doesn't work.
The websocket is closed with the "Sharding required" message.

How do you reproduce this bug

Log in as a bot that is part of 2k+ servers.

Error output

websocket: close 4011: Sharding required.

How to fix

The only way to solve this problem is to implement sharding.

[Bios-Marcel]

I actually thought discordgo already supports sharding. Maybe this just needs to be toggled.

[Bios-Marcel]

Appears there's no documentation about how the sharding exactly works with discordgo. Either way I don't want to implement this myself, as I have no way of testing this. I do not own any bots.

[Starwort]

I found this, it might be a simple dependency
https://github.com/jonas747/dshardmanager

[Bios-Marcel]

We'd need integration into https://github.com/Bios-Marcel/discordgo