Only types build option (or auto-detect)

[enisdenjo]

When building packages that contain only types (like @envelop/types), the resolution fields in package.json should contain:

{
  "main": "",
  "types": "index.d.ts"
}

without resolutions for module and other exports.

This aligns with how DefinitelyTyped packs types.

Additionally, this improvement would eradicate false positives for linting tools that check re-exports (like here).