Add snapshotting functionality

I would like to be able to create one or more snapshots of a lima vm and be able to roll back to some specified snapshot. I'm thinking this would be a copy of the qcow2 images stored in the `~/.lima/machine-name/` directory. It would be totally fine (and probably desirable) to shut the machine down before making a copy of the images.

This would be useful for e.g. testing a provisioning script. I could set up the machine with a base install of ubuntu, take a snapshot, and then be able to quickly roll back to the base state of the machine without necessarily re-doing the machine from scratch. It could also be useful for testing potentially destructive operations inside the VM.