Add ember-styleguide

Author: NullVoxPopuli

frontend/package.json

@@ -138,6 +138,7 @@
     "ember-router-helpers": "^0.4.0",
     "ember-select-light": "^2.0.5",
     "ember-statechart-component": "2.3.7",
+    "ember-styleguide": "6.1.2",
     "highlight.js": "^11.2.0",
     "highlightjs-glimmer": "^1.4.0",
     "html-to-image": "^1.7.0",