Experiment with runner scripts being replaced with one unified function (concrete)

We recently introduced user script migration to a Function. We may continue to do this for other scripts too.

However, a potentially faster and more practical way, given the code-reuse, and that the Functions we implement don't work well outside of the scripts-to-steps migration... is to have one unified Function, that encapsulates all of the existing Runner scripts.

This not only removes the abstract shell from the equation, it also places the hard-coded execution flow into a step.