fix: Ensure correct shard id when manually sharding.

Created by: Neuheit

Summary

Fixes #831 (closed)

Details

As mentioned in the issue, this is due to an oversight when the user is manually sharding, and won't calculate the correct value since it calculates the shard id from the gateway, rather than manually from the user. This PR fixes that by iterating through all guilds to find the shard id if they are manually sharding.