Bump cssnano from 5.1.12 to 5.1.13 in /csfieldguide

Bumps [cssnano](https://github.com/cssnano/cssnano) from 5.1.12 to 5.1.13.
- [Release notes](https://github.com/cssnano/cssnano/releases)
- [Commits](https://github.com/cssnano/cssnano/compare/cssnano@5.1.12...cssnano@5.1.13)

---
updated-dependencies:
- dependency-name: cssnano
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

csfieldguide/package.json

@@ -12,7 +12,7 @@
         "browserify": "17.0.0",
         "child_process": "1.0.2",
         "codemirror": "5.65.6",
-        "cssnano": "5.1.12",
+        "cssnano": "5.1.13",
         "details-element-polyfill": "2.4.0",
         "fancy-log": "2.0.0",
         "got": "11.8.2",