Ability to create multiple VMs in parallel

[jay7x]

While starting multiple VMs in parallel is unsafe in general (see lima-vm/lima#1354) it'd be really nice to have a way to 🤞🏻, 🙏🏻 and do it anyway. As long as all VMs are created already it should be safe to start them in parallel. So my idea is to add a parallel=true kind of parameter to the lima::cluster::start plan.

[jay7x]

Just realized that start process is already parallelized.. So two possible things to implement:

1. Add threads=N parameter to specify how many VMs to start in parallel (fallback to half of CPU count as it is now)
2. Add parallel=true which will create VMs in parallel too (if you're sure that all files were downloaded before)