Only ESM or only CJS build option

[n1ru4l]

Some packages only need cjs or esm. Thus it should be possible to specify that via the package.json config.

{
  "name": "fooo",
  "bob": {
    "cjs": false,
  }
}

Setting these options should also change the assertions for the package.json exports map. E.g. if "cjs" is false, a require should not be present in the exports map.