Implement slash command endpoints

Created by: IDoEverything

Summary

Adds all the slash command API endpoints

Details

Added all the documented endpoints, as well as the bulk overwrite endpoint, which hasn't been documented yet (for some reason, blame discord)(now documented). This will effectively mean that slash commands have been added to the library, but people will have to make their own handlers until we get around to making our own. I chose to add the guild endpoints to DiscordClient as well as DiscordGuild, since a lot of times you might not have the guild object and just the id, but this is up for discussion. I additionally also removed the restriction for editing files with webhooks.