Feature Request: Allow setting a minimum required concurrency for rules

[andrew-luminal]

For rules that are intended to spawn multiple open-ended processes (such as live-reloading rules), a certain level of concurrency is required, or they will break. Allow us to set a required_concurrency parameter on a rule? Behavior could either be that it fails with a message if the zim config jobs setting or the -j flag is too low, or just bump it up to that number for that rule.

Alternative option is to just have a live: true flag on rules that, when set, will automatically spawn that rule in its own process. This live: true flag could maybe also turn on --output unbuffered for that rule?