Fix EmojiUpdate passing the same collection

Created by: VelvetThePanda

Summary

GuildEmojiUpdate wouldn't actually send the before and after, but instead the same collection in both properties

Details

When guild emojis are updated, we simply dump the entire cache and iterate through what discord sends

But the issue is here. Creating a new ReadOnlyDictionary from an existing dictionary only wraps the passed dictionary, and thus when clearing the guild cache, it would clear the underlying dictionary because it's passed by ref and nothing more

Changes proposed

use .ToImmutableDictionary()

Notes

Someone bad