Update from webpacker v5 to shakapacker v6.0.0.rc13

Using a two-step process is recommended:
332e25186a/docs/v6_upgrade.md

config/webpack/webpack.config.js

@@ -0,0 +1,47 @@
+// See the shakacode/shakapacker README and docs directory for advice on customizing your webpackConfig.
+
+const { webpackConfig, merge } = require('shakapacker')
+const webpack = require('webpack');
+
+// Custom ERB loader to disable Spring and prevent crashes
+const erb = require("./loaders/erb");
+
+// This setting will change the absolute path used to refer
+// external files (images, fonts, ...) in the generated assets
+const relative_url_root = process.env.RAILS_RELATIVE_URL_ROOT || '';
+const public_output_path = webpackConfig.output.publicPath;
+
+const envConfig = module.exports = {
+    output: {
+        publicPath: relative_url_root + public_output_path
+    },
+    module: {
+        rules: [
+            erb
+        ]
+    },
+    resolve: {
+        alias: {
+            $: 'jquery/src/jquery',
+            jquery: 'jquery/src/jquery',
+        }
+    },
+    plugins: [
+        new webpack.ProvidePlugin({
+            $: 'jquery',
+            JQuery: 'jquery',
+            jQuery: 'jquery',
+            jquery: 'jquery',
+            'window.Tether': "tether",
+            Popper: ['popper.js', 'default'], // for Bootstrap 4
+            _: 'underscore',
+            vis: 'vis',
+            hljs: 'highlight.js',
+            d3: 'd3',
+            Sentry: '@sentry/browser',
+            Sortable: 'sortablejs',
+        })
+    ]
+}
+
+module.exports = merge(webpackConfig, envConfig)