Only ESM or only CJS build option

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

```json
{
"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.