Improve changelog (at least for major bumps)

[dontcallmedom]

we had two customers seeking clarity on the major bump of our latest @webref/css, which I take as a signal we should be more explicit about these :)
#688 (comment)
mdn/yari#6913 (comment)

[tidoust]

Right, an artefact of our release workflow is that the commit that bumps the version and that highlights the breaking change isn't directly visible when the package gets released.

Now, to improve the changelog, we would need to have a changelog in the first place ;)

[tidoust]

That's not perfect but we do have a changelog now that documents major version bumps in general, and breaking changes in particular.