add jquery (via webpack)

config/webpack/environment.js

@@ -1,3 +1,11 @@
 const { environment } = require('@rails/webpacker')
+const webpack = require('webpack')
+
+environment.plugins.prepend('Provide',
+  new webpack.ProvidePlugin({
+    $: 'jquery/src/jquery',
+    jQuery: 'jquery/src/jquery'
+  })
+)
 
 module.exports = environment