Add process polyfill for browsers

This was previously added by webpack4. It is needed in the form editor.
Solves error Uncaught ReferenceError: process is not defined
[MAILPOET-3214]

webpack.config.js

@@ -284,6 +284,9 @@ const adminConfig = {
         },
       ],
     }),
+    new webpack.ProvidePlugin({
+      process: 'process/browser',
+    }),
   ],
   optimization: {
     runtimeChunk: 'single',