Add /clear command

[Geczy]

Is your feature request related to a problem? Please describe.

I'm always frustrated when I have updates queued up, but I can't delete one

Describe the solution you'd like

Delete an update in the chat to prevent it from posting

Describe alternatives you've considered

/clear to delete all pending updates

Additional context

There's no event on deleting a message in a bot DM :( which makes native deletion hard but not impossible (can check if the msg id exists still / is viewable before it sends)