fix: resume lazy loading for Twitch videos (#1326)

The property of lazy load is missing in #1267

Author: cotes2020

_includes/embed/twitch.html

@@ -1,5 +1,6 @@
 <iframe
   class="embed-video twitch"
+  loading="lazy"
   src="https://player.twitch.tv/?video={{ include.id }}&parent={{ site.url | split: '://' | last | remove: '/' }}"
   frameborder="0"
   allowfullscreen="true"