Add support for node ESM addons

[hjdivad]

Node has support for packages that export files in ES module syntax. See module packages in the node documentation.

This feature doesn't quite work with ember addons due to in-addon files being loaded:

1. absolutely and
2. via require

Relative requires can be made to work with conditional exports, but even that would require addon authors to write those files in commonjs format.

Absolute requires cannot be made to work unless the file loaded has a cjs suffix.

This PR makes it possible to write ember addons that opt in to ES module syntax. The support is imperfect since config/environment.js still has to be in commonjs format, so in practice addon authors need to write:

// config/package.json
{
  "type": "commonjs"
}

[hjdivad]

@rwjblue obviously don't merge while there's a fixup! commit

I left it that way to make it easier to see the additional diff re the slow tests + watcher support