Add vue_shared_documentation Danger plugin

Adds plugin to check vue_shared files without
a Story.

danger/vue_shared_documentation/Dangerfile

+# frozen_string_literal: true
+
+def get_vue_shared_files(files)
+  files.select do |file|
+    file.end_with?('.vue') &&
+      file.include?('vue_shared/') &&
+      !File.file?(file.dup.sub!(/\.vue$/, '.stories.js'))
+  end
+end
+
+vue_shared_candidates = get_vue_shared_files(helper.all_changed_files)
+
+return if vue_shared_candidates.empty?
+
+warn 'This merge request changed undocumented Vue components in `vue_shared/`. Please consider [adding documention](https://docs.gitlab.com/ce/development/fe_guide/storybook).'
+
+if helper.ci?
+  markdown(<<~MARKDOWN)
+    ## Undocumented Vue components
+
+    The following Vue components don't have associated stories. Please consider [creating them](https://docs.gitlab.com/ce/development/fe_guide/storybook):
+
+    * #{vue_shared_candidates.map { |path| "`#{path}`" }.join("\n* ")}
+  MARKDOWN
+end

spec/tooling/danger/project_helper_spec.rb

@@ -221,7 +221,7 @@ RSpec.describe Tooling::Danger::ProjectHelper do
 
   describe '.local_warning_message' do
     it 'returns an informational message with rules that can run' do
-      expect(described_class.local_warning_message).to eq('==> Only the following Danger rules can be run locally: changelog, database, documentation, duplicate_yarn_dependencies, eslint, gitaly, karma, pajamas, pipeline, prettier, product_intelligence, utility_css')
+      expect(described_class.local_warning_message).to eq('==> Only the following Danger rules can be run locally: changelog, database, documentation, duplicate_yarn_dependencies, eslint, gitaly, karma, pajamas, pipeline, prettier, product_intelligence, utility_css, vue_shared_documentation')
     end
   end
 

tooling/danger/project_helper.rb

@@ -16,6 +16,7 @@ module Tooling
         prettier
         product_intelligence
         utility_css
+        vue_shared_documentation
       ].freeze
 
       CI_ONLY_RULES ||= %w[