Consider using exports field regardless of ESM

[niksy]

More expressive and standardised than main, module, browser and style.

• https://webpack.js.org/guides/package-exports/
• https://nodejs.org/api/packages.html#packages_package_entry_points
• Respect "exports" field in "package.json" webpack/webpack#9509
• Support "exports" field in package.json rollup/plugins#208

[niksy]

Since ESM packages are the only proper way forward and tooling is revolving around them, we should leave this as is and use only when producing ESM package.