Implement `/gurtignore` commands to manage channels Gurt should ignore.
- Add `gurtignore add` to add a channel to the ignore list.
- Add `gurtignore remove` to remove a channel from the ignore list.
- Add `gurtignore list` to display currently ignored channels.
- Update `GurtCog` and `config.py` to support ignored channel IDs and file management.
- Ensure ignore commands are owner-only.
- Modify `listeners.py` to prevent Gurt from processing messages in ignored channels.
Slipstream
3ff850a9b3
