configure webpacker builder to extract css into separate file

#343 (comment)

Author: fiedl

app/javascript/matestack-ui-core/index.js

@@ -7,6 +7,8 @@ import matestackEventHub from '../../../app/concepts/matestack/ui/core/js/event-
 import componentMixin from '../../../app/concepts/matestack/ui/core/component/component'
 import matestackUiCore from '../../../app/concepts/matestack/ui/core/js/core'
 
+import styles from './styles/index.css'
+
 const MatestackUiCore = {
   Vue,
   Vuex,

app/javascript/matestack-ui-core/styles/index.css

@@ -0,0 +1,3 @@
+body {
+  background: blue;
+}

builder/config/webpacker.yml

@@ -17,7 +17,7 @@ default: &default
   cache_manifest: false
 
   # Extract and emit a css file
-  extract_css: false
+  extract_css: true
 
   static_assets_extensions:
     - .jpg