Github Workflow that checks for CHANGELOG, breaking changes on PRs to published packages

It is easy to forget to add a `CHANGELOG` entry or introduce a breaking change to the packages in the DevTools repo that are published on pub (e.g. `devtools_shared`, `devtools_app_shared`).

We should add a Github workflow/s that can detect breaking changes and remind authors to add `CHANGELOG` entries, similar to the release notes check we currently have in place.

I think we could use https://github.com/dart-lang/ecosystem/tree/5c8a298e4042ebdc0819441036adac7ce4842c05/pkgs/firehose for the breaking changes check.