Part 3 of moving off oclif to vanilla CLI

[scalvert]

This one is a big one...

This is the port of oclif to vanilla CLI by implementing the following:

1. A new CLI via yargs - checkup run ...
2. A new Node API implemented in the CheckupTaskRunner class, which encapsulates the functionality that was previously in the RunCommand
3. Extensive updates to the generators such that we now utilize the generators for generating plugin and task test fixtures. This provides a powerful capability of ensuring that the JavaScript generated files are still functional, while our existing plugins test the TypeScript code paths
4. Tests have been changed to use execa to execute the bin script directly, while the node API tests allow us to test the API directly, without the expensive overhead of creating fixtures and executing via execa.

This PR may have a few changes along the way, but I wanted to get it out there for folks to start commenting on.

[carakessler]

Cursory review looks good, just a few questions!

[scalvert]

@carakessler @rwjblue since this is largely a PR that we've collectively discussed/worked on together, and in the interest of moving forward with the broad changes targeted for v1.0.0, I'm going to merge this in. We can absolutely address/revisit anything we want to change later.