introduce importmaps (#983)

* introduce importmaps

This commit introduces importmaps. They allow to use modern javacript ESM within rails without webpack, yarn etc.
see https://github.com/rails/importmap-rails for more details.

Co-authored-by: Philipp Rothmann <philipprothmann@posteo.de>
Co-authored-by: FGU <fgu@pragma-shift.net>

* fix: rubocop violations

---------

Co-authored-by: FGU <fgu@pragma-shift.net>

app/javascript/application.js

@@ -0,0 +1 @@
+// Configure your import map in config/importmap.rb. Read more: https://github.com/rails/importmap-rails

app/views/layouts/_header.html.haml

@@ -8,10 +8,10 @@
     = csrf_meta_tags
     = stylesheet_link_tag "application", :media => "all"
     //%link(href="images/favicon.ico" rel="shortcut icon")
-
     = yield(:head)
     = foodcoop_css_tag
 
+
   %body
     = yield
 
@@ -19,7 +19,9 @@
       Javascripts
       \==================================================
     / Placed at the end of the document so the pages load faster
-    = javascript_include_tag "application"
+    = javascript_importmap_tags
+    = javascript_include_tag "application_legacy"
+
     :javascript
       I18n.defaultLocale = "#{I18n.default_locale}";
       I18n.locale = "#{I18n.locale}";