Set webpack configuration.devtool to 'source-map' instead of 'sourcemap' (#662)

Webpack 5 is more strict, so we have to use 'source-map'.

Author: marco-c

addon/webpack.config.js

@@ -26,5 +26,5 @@ module.exports = {
 
   // This will expose source map files so that errors will point to your
   // original source files instead of the transpiled files.
-  devtool: 'sourcemap',
+  devtool: 'source-map',
 };