chore: restore missing dependency and build files in node_modules

node_modules/postcss/lib/stringify.js

@@ -0,0 +1,11 @@
+'use strict'
+
+let Stringifier = require('./stringifier')
+
+function stringify(node, builder) {
+  let str = new Stringifier(builder)
+  str.stringify(node)
+}
+
+module.exports = stringify
+stringify.default = stringify