feat: no longer write json unless configured

BREAKING CHANGE: No json coverage is generated by default. If you want that, please use `json` reporter:

```js
coverageReporter: {
  type: 'json'
}

// or with multiple reporters
coverageReporter: {
  reporters: [
    {type: 'html', dir: 'coverage'},
    {type: 'json', dir: 'coverage'},
  ]
}
```