Add missing bot-to-bot DM sanity checks
Created by: uwx
Summary
I noticed these were missing when looking over the code. This makes the rest of the DiscordMember direct message methods fail fast when a bot attempts to DM another bot.
Details
Simply copied the checks from the methods that had them, to the methods that didn't.
Changes proposed
- Assert that the current user and the target member are not both bots in DiscordMember.SendFileAsync and SendMultipleFilesAsync.
Notes
I didn't test this PR.