Internationalisation of vue_shared directory

This is one of many MRs opened in order to improve the overall
internationalisation of the GitLab codebase.

This commit only targets the Vanilla JS files.

i18n documentation
https://docs.gitlab.com/ee/development/i18n/externalization.html

app/assets/javascripts/vue_shared/components/content_viewer/lib/viewer_utils.js

+import { __ } from '~/locale';
+
 const viewers = {
   image: {
     id: 'image',
   },
   markdown: {
     id: 'markdown',
-    previewTitle: 'Preview Markdown',
+    previewTitle: __('Preview Markdown'),
   },
 };
 

locale/gitlab.pot

@@ -6688,6 +6688,9 @@ msgstr ""
 msgid "Preview"
 msgstr ""
 
+msgid "Preview Markdown"
+msgstr ""
+
 msgid "Preview changes"
 msgstr ""