make own dummy, less confusing what is gem/engine and what is app

dummy/app/assets/config/manifest.js

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+//= link_tree ../images
+//= link_directory ../stylesheets .css
+//= link_directory ../javascript .js

dummy/app/assets/javascript/application.js

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+//= require opal_main
+
+Opal.require('opal_main');

dummy/app/assets/javascript/opal_main.js.rb

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+require "vue_r"

dummy/app/assets/stylesheets/application.css

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+/*
+ * This is a manifest file that'll be compiled into application.css, which will include all the files
+ * listed below.
+ *
+ * Any CSS (and SCSS, if configured) file within this directory, lib/assets/stylesheets, or any plugin's
+ * vendor/assets/stylesheets directory can be referenced here using a relative path.
+ *
+ * You're free to add application-wide styles to this file and they'll appear at the bottom of the
+ * compiled file so the styles you add here take precedence over styles defined in any other CSS
+ * files in this directory. Styles in this file should be added after the last require_* statement.
+ * It is generally better to create a new file per style scope.
+ *
+ *= require_tree .
+ *= require_self
+ */