Introduce change log enforcement via CI

[chrismarget-j]

This PR introduces scripts and a CI workflow which ensure that PRs include appropriate updates to the change log.

There are three scripts:

• scripts/changie_pr_check.sh runs exactly one of these two:
  • scripts/changie_normal_pr_check.sh
  • scripts/changie_release_pr_check.sh

Whether a change is a "normal" or a "release" PR depends on the label, probably over here somewhere =====>>>

PRs labeled with release (case insensitive) are subjected to "release" review:

• Change log generated from fragments in ./changes/unreleased/
• No change fragments remaining in ./changes/unreleased/
• New change log has appropriate version for release (calculated by changie based on accumulated change fragments)

All other PRs are subject to "normal" review:

• Exactly one new change fragment required in ./changes/unreleased/ (generate with changie new)
• No other changes permitted to changie files