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

spec/models/topic_embed_spec.rb

@@ -390,10 +390,10 @@ describe TopicEmbed do
     it "handles malformed links" do
       url = "https://somesource.com"
 
-      contents = <<~CONTENT
+      contents = <<~HTML
       hello world new post <a href="mailto:somemail@somewhere.org>">hello</a>
       some image <img src="https:/><invalidimagesrc/">
-      CONTENT
+      HTML
 
       raw = TopicEmbed.absolutize_urls(url, contents)
       expect(raw).to eq(contents)