CPP-2161 parallel tasks

[apaleslimghost]

implements parallel task running in userland using a task with options, which looks like:

commands:
  run:local:
    - Parallel:
        tasks:
          - Node:
              entry: server/index.js
          - Webpack:
              watch: true

also includes some refactoring to the core that was necessary to get this working, and some refactoring that turned out not to be necessary.

[olusoladavid]

Great work! Haven't had the chance to fully test the new UX but the code approach is solid 🔥 . Thanks for putting together. I added some questions in the meantime

[olusoladavid]

sheer brilliance; thank you - there's the DX edge case I mentioned but I wouldn't consider it blocking for an initial rollout of this feature