Bump @bookshop/jekyll-engine from 3.8.1 to 3.9.0 (#194)

Bumps [@bookshop/jekyll-engine](https://github.com/cloudcannon/bookshop/tree/HEAD/engines/jekyll-engine) from 3.8.1 to 3.9.0.
- [Release notes](https://github.com/cloudcannon/bookshop/releases)
- [Commits](https://github.com/cloudcannon/bookshop/commits/v3.9.0/engines/jekyll-engine)

---
updated-dependencies:
- dependency-name: "@bookshop/jekyll-engine"
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Jessica Brentnall <[email protected]>

package.json

@@ -23,7 +23,7 @@
   "devDependencies": {
     "@bookshop/browser": "3.8.1",
     "@bookshop/live": "3.8.1",
-    "@bookshop/jekyll-engine": "3.8.1",
+    "@bookshop/jekyll-engine": "3.9.0",
     "npm-run-all": "^4.1.5"
   },
   "dependencies": {