Add postinstall script to output how to use it

`eleventy-sass` uses an experimental feature of Node.js 22 and users
must add `--experimental-require-module` option to their Node commands.

That is a breaking change and users must be notified of it.

package.json

@@ -4,6 +4,7 @@
   "description": "Yet another Sass/SCSS plugin for Eleventy. Source maps, PostCSS and revision hashes are supported.",
   "main": "index.js",
   "scripts": {
+    "postinstall": "node scripts/postinstall.js",
     "test": "npx ava --timeout=3m --concurrency=1"
   },
   "repository": {

scripts/postinstall.js

@@ -0,0 +1,6 @@
+const { yellow, green } = require("kleur");
+
+console.log(yellow("`eleventy-sass` uses an experimental feature of Node 22 to load Eleventy 3 internal modules. You should add \"--experimental-require-module\" option to your Node commands. Therefore, instead of running `npx @11ty/eleventy`, use the following command:"));
+console.log(green("npx --node-options=\"--experimental-require-module\" @11ty/eleventy"));
+console.log("HOGE");
+console.debug("FOO");