Move codebase over to ESM

[jasonkuhrt]

This issue is to track moving the codebase toward pure ESM. Achieving this will surely require multiple sub-tasks to incrementally and systematically achieve the refactor.

[kamilkisiela]

What will be the benefit here?

[jasonkuhrt]

There is more than one answer to that question. We can track them in this issue over time as we run up against use-cases.

[jasonkuhrt]

We can drop the esm2cjs/execa wrapper package in favour of the package directly.