Ability to create multiple VMs in parallel

While starting multiple VMs in parallel is unsafe in general (see https://github.com/lima-vm/lima/issues/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.

Start by reading the lima::cluster::start plan and the referenced Lima issue #1354 to understand the safety constraint. Define how a parallel=true parameter should behave after all VMs are created, and verify that the plan starts those VMs concurrently without affecting the default behavior.

Written by the indexing model from the issue text.