ci: fail the pull request whose changeset would generate an unformatted changelog

[bendersej]

Background

The version PR #53 failed Web / Formatting: the 1.9.0 changeset has a blank line between its lead paragraph and its list, which changeset version turns into a blank line inside a list item that Prettier rejects. The failure only showed in the version PR, after the feature PR had merged. The version PR should only ever contain generated output, never a formatting fix.

Changes

• The root version script is a pure changeset version (no formatter), for every package.
• New Changelog Check workflow on pull requests touching a package, a changeset or the root package.json: it runs the release's npm run version, then the formatting checks the version PR runs, so a changeset whose generated CHANGELOG is not formatted fails the pull request that adds it.
• The pending 1.9.0 changeset: the blank line before its list is removed.

Notes

• Verified locally with the pure version script: the original 1.9.0 changeset fails the check (as chore: version packages #53 did) and the fixed one passes.
• Once merged, the release workflow rebuilds chore: version packages #53 from the fixed changeset, with nothing to reformat.