Upgrade next.js to 12 and related packages (#5042)

* Upgrade next.js and source-map-loader

* Add changeset

* Upgrade eslint and typescript

* improve reliability of iframe test

* fix lint

site/next.config.js

@@ -5,9 +5,13 @@ module.exports = {
   webpack: config => {
     config.module.rules.push({
       test: /\.js$/,
-      loader: require.resolve('source-map-loader'),
       enforce: 'pre',
       exclude: [/node_modules\/@next/, /node_modules\/next/],
+      use: [
+        {
+          loader: require.resolve('source-map-loader'),
+        },
+      ],
     })
     return config
   },