build: add JS and CSS linters

.eslintrc.js

@@ -0,0 +1,12 @@
+module.exports =  {
+  env: {
+  	browser: true,
+  	jquery: true,
+  	es6: true
+  },
+  extends:  [
+    'wordpress',
+    'plugin:prettier/recommended',  // Enables eslint-plugin-prettier and displays prettier errors as ESLint errors. Make sure this is always the last configuration in the extends array.
+  ],
+  rules:  {},
+};