move postcss functions to own module

static-html/postcss.config.js

@@ -1,6 +1,4 @@
-function half(val) {
+const postcssFunctions = require('./src/js/postcss-functions.js');
-  return val / 2;
-}
 
 module.exports = {
   plugins: [
@@ -11,7 +9,7 @@ module.exports = {
     'postcss-simple-vars',
     ['postcss-functions',
       {
-        functions: { half },
+        functions: postcssFunctions,
       },
     ],
   ],

static-html/src/js/postcss-functions.js

@@ -0,0 +1,7 @@
+const postcssFunctions = {
+  half(val) {
+    return val / 2;
+  },
+};
+
+module.exports = postcssFunctions;