feat: enable custom packages encapsulation

[mallachari]

Needed by stoplightio/elements#1392

Having exports property set to:

{
  "require": "./index.js",
  "import": "./index.mjs"
}

encapsulates exported packages preventing any other to be used. This PR allows setting custom packages by defining them in the original package.json file.

This way using

"exports": {
  "./styles.min.css": "./styles.min.css"
}

results in

"exports": {
  "./styles.min.css": "./styles.min.css",
  ".": {
    "require": "./index.js",
    "import": "./index.mjs"
  }
}

[mpodlasin]

Looks good!

[stoplight-bot]

🎉 This PR is included in version 9.1.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀

[marcelltoth]

Awesome work @mallachari !