Convert taxonomy_term_feed() to a new style controller

First attempt.

+++ b/core/modules/taxonomy/lib/Drupal/taxonomy/Controller/TaxonomyTermController.phpundefined
@@ -0,0 +1,36 @@
+    $channel['title'] = config('system.site')->get('name') . ' - ' . $taxonomy_term->label();

this needs to be injected, inject config.factory and set $this->config = $config_factory->get('system.site') then use $this->config->get('name')

+++ b/core/modules/taxonomy/lib/Drupal/taxonomy/Controller/TaxonomyTermController.phpundefined
@@ -0,0 +1,36 @@
+    $nids = taxonomy_select_nodes($taxonomy_term->id(), FALSE, config('system.rss')->get('items.limit'));

This really needs to go to the Taxonomy service, please add a todo for when 1976298 is in

i think this is going to be a view after #1857256: Convert the taxonomy listing and feed at /taxonomy/term/%term to Views lands
i am not going to postpone this, since the issue above seems stuck for now

actually, the issue above contains the conversion already