Fail a PR if it changes source code without a changelog entry

Spotless Changelog can handle the case where a single git repository has multiple changelogs for its various components. However, it can be easy to miss when a changelog which is mostly about component A, actually also brushes againt component B, and it can be bad if you forget to add a changelog for component B.

Perhaps we should implement something like the following:

```gradle
spotlessChangelog {
ifAnyFilesChangedIn('./lib/**/*', './lib-extra/**/*').since('origin/main').thenEntryRequired()
...
```

The snippet above could have avoided a problem [we had recently in spotless](https://github.com/diffplug/spotless/pull/706#issuecomment-703397058).