test: percy visual testing (#1053)

* test: percy visual testing init

* Try PR checks with custom command

* Only run percy visual tests for PWA on merge for now

.webpack/webpack.base.js

@@ -1,3 +1,6 @@
+// ~~ ENV
+const dotenv = require('dotenv');
+//
 const path = require('path');
 const webpack = require('webpack');
 const PACKAGE = require('../platform/viewer/package.json');
@@ -12,6 +15,9 @@ const NODE_ENV = process.env.NODE_ENV;
 const QUICK_BUILD = process.env.QUICK_BUILD;
 const BUILD_NUM = process.env.CIRCLE_BUILD_NUM || '0';
 
+//
+dotenv.config();
+
 module.exports = (env, argv, { SRC_DIR, DIST_DIR }) => {
   if (!process.env.NODE_ENV) {
     throw new Error('process.env.NODE_ENV not set');