Migrate CSS language to ES Module (#12341)

scripts/build/config.mjs

@@ -121,6 +121,7 @@ const parsers = [
       // https://github.com/shellscape/postcss-values-parser/blob/c00f858ab8c86ce9f06fdb702e8f26376f467248/lib/parser.js#L499
       "node.constructor.name === 'Word'": "node.type === 'word'",
     },
+    isEsm: true,
   },
   {
     input: "src/language-graphql/parser-graphql.js",