Add `exports` map with ESM variant

[lukeed]

It'd be nice to offer a supplemental ESM build that's linked via:

• "module" in package.json
• the import key inside each exports entry (inside package.json too)

This'd end up looking something like:

{
  // ...
  "module": "dist/index.mjs",
  "main": "dist/index.js",
  // ...
  "exports": {
    ".": {
      "require": "./dist/index.js",
      "import": "./dist/index.mjs"
    },
    "./from": {
      "require": "./from/index.js",
      "import": "./from/index.mjs"
    }
  }
  // ...
}

This allows for native ESM resolution in Node.js (12.x and >=14) in supporting environments w/o sacrificing CommonJS (default) support. Also allows bundler(s) to pick up the ESM entry, too.

There's one gotcha – Node 13.0 thru 13.7 don't support this version of exports.
This can be patched – but only if you decide to keep the default exports (module.exports / export default) that you currently have. This means that the the addition remains non-breaking, even w/ these Node versions.

---

I can PR these changes if desired.

[jimmywarting]

The v3 have switched to ESM - no longer provide a commonjs variant