Create an htmx_entity_view_field route

I don't have time yet to create test but just wanted to get some code up for proposal what I think is a useful feature and get feedback on the idea.

Thank you @apmsooner.

I am not sure about returning full entity as fallback. Returning empty string as fallback looks more suitable to me. Returning full entity might break things while empty string can be handled by devs as long as they know what they are requesting for.

      // Render the full entity as fallback.
      else {
        $build = $viewBuilder->view($entity, $viewMode);
      }

Nice! We need a test and to clean up some lint