Bump reffy from 18.4.7 to 18.6.0

Bumps [reffy](https://github.com/w3c/reffy) from 18.4.7 to 18.6.0.
- [Changelog](https://github.com/w3c/reffy/blob/main/CHANGELOG.md)
- [Commits](https://github.com/w3c/reffy/commits)

---
updated-dependencies:
- dependency-name: reffy
  dependency-version: 18.6.0
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

package-lock.json

@@ -35,7 +35,7 @@
     "@webref/idl": "file:packages/idl",
     "css-tree": "3.1.0",
     "mocha": "11.1.0",
-    "reffy": "18.4.7",
+    "reffy": "18.6.0",
     "rimraf": "6.0.1",
     "strudy": "^3.1.1",
     "webidl2": "24.4.1"