Allow to import additional admin JS modules

In order to be able to import additional JS modules
into the Alchemy admin UI you can use this.

Be sure to also pin the modules with `Alchemy.importmap`.

\## Example

    Alchemy.importmap.pin "flatpickr/de",
      to: "https://ga.jspm.io/npm:flatpickr@4.6.13/dist/l10n/de.js"

    Alchemy.admin_js_imports << "flatpickr/de"

app/views/layouts/alchemy/admin.html.erb

@@ -35,6 +35,13 @@
     <%= render 'alchemy/admin/partials/routes' %>
     <%= javascript_include_tag('alchemy/admin/all', 'data-turbo-track' => true) %>
     <%= javascript_importmap_tags("alchemy_admin", importmap: Alchemy.importmap) %>
+    <% if Alchemy.admin_js_imports.any? %>
+      <script type="module">
+        <% Alchemy.admin_js_imports.each do |path| %>
+          import "<%= path %>"
+        <% end %>
+      </script>
+    <% end %>
     <%= yield :javascript_includes %>
   </head>
   <%= content_tag :body, id: 'alchemy', class: alchemy_body_class do %>

lib/alchemy.rb

@@ -43,6 +43,25 @@ def self.preview_sources=(sources)
     @_preview_sources = Array(sources)
   end
 
+  # Additional JS modules to be imported in the Alchemy admin UI
+  #
+  # Be sure to also pin the modules with +Alchemy.importmap+.
+  #
+  # == Example
+  #
+  #    Alchemy.importmap.pin "flatpickr/de",
+  #      to: "https://ga.jspm.io/npm:flatpickr@4.6.13/dist/l10n/de.js"
+  #
+  #    Alchemy.admin_js_imports << "flatpickr/de"
+  #
+  def self.admin_js_imports
+    @_admin_js_imports ||= Set.new
+  end
+
+  def self.admin_js_imports=(sources)
+    @_admin_js_imports = Set[sources]
+  end
+
   # Define page publish targets
   #
   # A publish target is a ActiveJob that gets performed