DEV: Correctly tag heredocs (#16061)

This allows text editors to use correct syntax coloring for the heredoc sections.

Heredoc tag names we use:

languages: SQL, JS, RUBY, LUA, HTML, CSS, SCSS, SH, HBS, XML, YAML/YML, MF, ICS
other: MD, TEXT/TXT, RAW, EMAIL

plugins/lazy-yt/plugin.rb

@@ -36,7 +36,7 @@ class Onebox::Engine::YoutubeOnebox
       # Put in the LazyYT div instead of the iframe
       escaped_title = ERB::Util.html_escape(video_title)
 
-      <<~EOF
+      <<~HTML
       <div class="onebox lazyYT lazyYT-container"
            data-youtube-id="#{video_id}"
            data-youtube-title="#{escaped_title}"
@@ -49,7 +49,7 @@ class Onebox::Engine::YoutubeOnebox
                title="#{escaped_title}">
         </a>
       </div>
-      EOF
+      HTML
     else
       yt_onebox_to_html
     end