Upgrade @csstools/postcss-light-dark-function to version 3.0.0

This is a major version bump, but the changelog at
https://github.com/csstools/postcss-plugins/blob/main/plugins/postcss-light-dark-function/CHANGELOG.md
doesn't indicate any breaking changes that should impact us.

We only have to list `@csstools/postcss-light-dark-function` in
`eslint.config.js` similar to e.g. commit a2909f9.

Author: timvandermeij

eslint.config.mjs

@@ -116,6 +116,7 @@ export default [
             "pdfjs-lib",
             "pdfjs-web",
             "web",
+            "@csstools/postcss-light-dark-function",
             "fluent-bundle",
             "fluent-dom",
             "postcss-dir-pseudo-class",

package-lock.json

@@ -5,7 +5,7 @@
     "@babel/core": "^7.29.0",
     "@babel/preset-env": "^7.29.0",
     "@babel/runtime": "^7.28.6",
-    "@csstools/postcss-light-dark-function": "^2.0.11",
+    "@csstools/postcss-light-dark-function": "^3.0.0",
     "@fluent/bundle": "^0.19.1",
     "@fluent/dom": "^0.10.2",
     "@metalsmith/layouts": "^3.0.0",