Should we have a `!mjolnir mute` command?

I know that the same can be achieved using the `!mjolnir powerlevel` command but having a dedicated mute command is easier, this would also be necessary if we use membership state in the future to track mutes instead of power levels (MSC3909).

May help with fixing #62

Start by reading the existing `!mjolnir powerlevel` command and the discussion in issue #62. Determine the intended syntax and behavior for a dedicated mute command, including how it would relate to future membership-state mutes under MSC3909. Done means the command provides the proposed muting workflow without requiring the equivalent power-level command.

Written by the indexing model from the issue text.