Add a way to verify that each changelog entry has an associated PR

[mcmire]

Currently, auto-changelog treats the PR reference at the end of a changelog entry as optional. This was done intentionally, and in many cases this is fine. But the core monorepo has many contributors who may not be aware of all of the guidelines around changelogs, so in order to keep those changelogs as useful as possible, we want auto-changelog to be more strict. We should add an option to this package that, when enabled, will check for the existence of PR references and fail validation if one is not supplied.