Pull Request: Native ES Module support for Oclif w/ continued support of CommonJS

[typhonrt]

Greets,

I have to say I have been enjoying working with Oclif. It is going to allow me to complete a complex CLI project soon. What would be even better is being able to work with ES Modules (ESM) for the core CLI and plugin development. I personally along with folks like Sindre Sorhus are moving to ESM for all new Node releases imminently. Tooling needs to play catchup, but I like Oclif so much I spent a full day updating Oclif to support ESM along with maintaining CommonJS support. With my pull request it is possible to write core Oclif CLI code with ESM along with loading a mixture of ESM / CJS Oclif plugins. As noted at the bottom of the pull request I put up a separate Github repo w/ the compiled Typescript code that can be linked from Github to replace @oclif/config enabling ESM support now.

I look forward to working on this pull request and getting things on the up and up for Oclif w/ ESM support.

Apologies for the double issue post here and in @oclif issues. I just figure posting there as well might reach a few more eyes in general.

[typhonrt]

I've decided a single posted issue is easier to track. So please refer to the issue in the main oclif issue tracker.
oclif/oclif#521