feat: change whitelistPatterns to safelist

The options naming for this packages has changed https://github.com/FullHuman/purgecss/tree/master/packages/postcss-purgecss
it no longer accepts `whitelistPatterns` key but accepts `safelist` instead.

I spent some time debugging an issue with styles not being shipped with prod environment, maybe this change will help someone avoid that :D

Author: BobrImperator

docs/TAILWINDCSS.md

@@ -52,7 +52,7 @@ const purgecssOptions = {
   // Leave css definitions starting with an underscore untouched.
   // This prevents css definitions generated by ember-css-modules
   // from being purged.
-  whitelistPatterns: [/^_/]
+  safelist: [/^_/]
 };
 
 let cssModulesPlugins = [