Prioritize one of post tags

[dziudek]

It will be useful e.g. for breadcrumbs and SEO (proper SERPs).

ref. https://publii.ticksy.com/ticket/1899413/

[dziudek]

Example:

               {{#if mainTag}}
               <div>
                  <a href="{{ mainTag.url }}">
                     MAIN: {{ mainTag.name }}
                  </a>
               </div>
               {{/if}}

Important! mainTag is empty only if the post has no tags. If the main tag is not set the main tag is set to the first tag of the post (using alphabetical order).