Add rake task to vendor guide javascript

The Rails guides were migrated from Turbolinks to Turbo in
0f6575a

The PR discussion explains that the file was grabbed from unpkg, and a
decision was made to not run it through a minifier.

This commit adds a rake task to automate the process of grabbing the
turbo js file using Importmap::Packager, which is what importmap-rails
uses for `bin/importmap pin --download`.

The extra Importmap module definition is necessary because the
Importmap::Packager file uses the shorthand module syntax, meaning that
an error is thrown if the Importmap module is not previously defined.
`require "importmap-rails"` would normally define this module, but one
of its dependent requires will not load outside of a Rails application.

The turbo.css file was removed as it appears to be leftover from
Turbolinks and Turbo does not provide any css files in its dist.

guides/Rakefile

@@ -30,6 +30,19 @@ namespace :guides do
     ruby "w3c_validator.rb"
   end
 
+  task :vendor_javascript do
+    module Importmap; end
+    require "importmap/packager"
+
+    packager = Importmap::Packager.new(vendor_path: "assets/javascripts")
+    imports = packager.import("@hotwired/turbo", from: "unpkg")
+    imports.each do |package, url|
+      umd_url = url.gsub("esm.js", "umd.js")
+      puts %(Vendoring "#{package}" to #{packager.vendor_path}/#{package}.js via download from #{umd_url})
+      packager.download(package, umd_url)
+    end
+  end
+
   desc "Show help"
   task :help do
     puts <<HELP