[feature] explicit serial/parallel task execution control

[theoephraim]

Is your feature request related to a problem? Please describe.

Not a huge issue, but in my use case, I have a task that I'd like to call before a set of other tasks in some situations. It's not necessarily a strict dependency, but maybe it would affect caching or perform some setup. I'd like to be able to describe a flow where I say, run this thing first, then these tasks in parallel.

Describe the solution you'd like

When opting out of the automatic parallelism that is controlled purely by task dependencies, it would be nice to still have a bit more control than "run these tasks serially".

I can imagine a very simple solution using nested arrays, for example:

tasks:
  foo:
    options:
      runDepsInParallel: false
    dependsOn:
      - 'task-a'
      - ['task-b1', 'task-b2', 'task-b3']
      - 'task-c'

      # (or the yml way of doing nested lists)
      - 'task-a'
      - - 'task-b1'
        - 'task-b2'
        - 'task-b3'
      - 'task-c'

This would run task A, then all the B tasks in parallel, then C.

Describe alternatives you've considered

You may be able to accomplish this by breaking the task up in to multiple subtasks, but it kind of pollutes your config if those subtasks are not meant to be called on their own