Added source-map-loader to the Next Webpack config to pass through the source maps from the compiles Slate libraries. (#3445)

site/next.config.js

@@ -20,4 +20,12 @@ module.exports = {
 
     return pages
   },
+  webpack: config => {
+    config.module.rules.push({
+      test: /\.js$/,
+      use: ['source-map-loader'],
+      enforce: 'pre',
+    })
+    return config
+  },
 }