Add schedule message command

[Defelo]

• Add a .schedule command with the same subcommands as .send and .edit that that automatically sends a message at a specified point in time.
• You should be able to specify both an absolute timestamp (e.g. 13:37:42, 2021-11-01 13:37:42) and a relative timedelta (e.g. 20s, 10m, 2h, 7d).
• Add a command to list all active schedules.
• Add commands to edit an active schedule (timestamp, message, channel)
• Add a command to immediately send the message of an active schedule
• Add a command to delete an active schedule

easier than YT-search xD