Add Yarn build script that builds CSS and JS

Signed-off-by: Maurício Meneghini Fauth <mauricio@fauth.dev>

package.json

@@ -53,7 +53,8 @@
     "lodash": "^4.17.19"
   },
   "scripts": {
-    "postinstall": "yarn run --silent css-compile --quiet --style=compressed && yarn run --silent js-compile --quiet",
+    "postinstall": "yarn run build",
+    "build": "yarn run css-compile --style=compressed && yarn run js-compile",
     "css-compile": "sass themes/pmahomme/scss:themes/pmahomme/css themes/original/scss:themes/original/css themes/metro/scss:themes/metro/css themes/bootstrap/scss:themes/bootstrap/css",
     "css-lint": "stylelint --syntax scss \"themes/**/scss/*.scss\"",
     "js-lint": "eslint js/src",